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InläggPostat: lör aug 27, 2011 11:36 

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Tire Sizing Systems

There are many tire sizing systems still in use today, some dating back to the 1930’s and others having come into use only since the 70’s and 80’s. I will explain each one, as you are likely to see any and all of these systems when shopping for off-road tires (Interco themselves use four different systems: Alpha-Numeric, Metric, Flotation, and LT-Numeric).


Earliest system, dates back to the 30’s when there were few tire sizes available (or needed)



9.15 is the section width in inches
15 is the rim diameter in inches

- Using this system you can’t tell the OD of the tire without knowing the Aspect Ratio
- In the 1950s and early 1960s, the standard aspect ratio was 82-84%
- Around 1970, bias ply tires moved to a 78% aspect ratio
- In the 70’s the "standard" aspect ratio dropped to 75%
- Today, Aspect Ratios of 65%, 60% and even as low as 35% are available.
- If you know the AR of the tire in question you can calculate the OD as: (Section Width x Aspect Ratio) x 2 + Rim Diameter (eg. in the above example, if we knew the Aspect Ratio were 78%: (9.15 x 0.78) x 2 + 15 = 29.3" )
- If you don’t know the AR, the only way to tell the OD is to consult the manufacturer’s specifications.

Alpha Numeric

This system began to appear in the late 1960’s. It adds the Load Rating and Aspect Ratio information, but mysteriously drops the Section Width.



H – load rating
78 – Aspect ratio in %
15 – diameter in inches

– This is an almost useless system as we don’t know, nor can we determine, the section width or OD of the tire.
– When radial tires began to appear, an R was added to the designation but this still doesn't’t help us determine size:



– There are some charts available that list common Alpha-Numeric sizes and the equivalent OD and Section Width of the tire, in inches, but these must be used with caution as they are by no means universal. Here is one such chart:

L78/15 = 30x 9.5
N78/15 = 31x 9.5
P78/15 = 33x10.0
Q78/15 = 36x11.5
Q78/16 = 36x10.5
R78/15 = 37x12.5
R85/16 = 37x12.0

-Though the alpha-numeric system doesn’t tell us the OD, we can, however, tell the relative size of tires using this system. Here’s why:

There are only two ways to increases the load carrying capacity of a tire

1) Increase the max pressure it can hold (by altering its construction/composition) or
2) Increase the size of the tire so there are more Square Inches for each Pound of air pressure to push against (a tires load in Pounds is roughly it’s max pressure (PSI) multiplied by the number of Square Inches of its contact patch)

E.g. Load = 50 P/SI x 30 SI = 1500 lbs

The alphabetic load rating system was originally based on this principle, with higher letters being assigned to larger tires able to carry more load. For this reason, we can also say that, for tires using the alpha-numeric system, the higher the letter, the larger the tire. For example, we can see from the preceding chart that an N78/15 (31x9.5) is smaller than a Q78/15 (36x11.5)
- Again, the best way to determine the dimensions of an Alpha-Numeric tire is to consult the manufacturer’s specifications.

Metric (Euro-Metric, P-Metric)

The Metric system first appeared in the 1970’s and was called the Euro-Metric system because it began in Europe. It is the first system that tells us virtually all we need to know, though some information must be calculated from the figures given.



235 is the section width in mm
75 is the Aspect Ratio in %
R denotes radial construction (D = (diagonal) bias ply construction; B = belted bias construction)
15 is the rim diameter in inches

To calculate the complete dimensions:

First change section width to inches by dividing the mm by 25.4

235/25.4 = 9.25” wide

Then calculate the OD in inches by multiplying the Section Width by the Aspect Ratio (to get the height of the sidewall), doubling it, and adding the diameter of the rim

OD = ((9.25*.75)*2)+15 = 28.87 ~ 29”

Therefore a 235/75/R15 is a 29” x 9.25” R15


- This system has becom widely known as the “P-Metric” system as a P appears at the beginning of modern designations to indicate a “Passenger car” tire. Although the system is also used for light truck and special trailer tires.


LT 275/70R16
ST 205/75R14

- System allows rapid comparison of the AR or “profile” of the tire
- Unlikely to ever see anything but an R in P and LT tires, but may see a D or B for certain special tires like ST tires
- The D.O.T. decided to add the "P" to P-Metric to denote that it was designed for passenger car use for load carrying capacity limits.
- Note that P and LT tires are often rated slightly different. If P-rated tire is used in an LT application, you need to de-rate the sidewall load capacity by 9% - 10% according to many tire makers.
- The best system so far, but not perfect (for us) as it requires calculations to give the complete dimensions which we need (because we run such a VAST range of different sizes)
- Early metric sizes may be shown without the AR number



This is because the metric system was implemented in Europe when tires were still chiefly bias ply, available only in a limited number of sizes (due to construction technology of the time) and all using a standard aspect ratio of "82". Therefore, originally it was not necessary to include the AR in the sizing system. With the advent of radial tire construction, tire manufacturer’s were suddenly able to construct better performing street tires by increasing the section width and reducing the sidewall height of the tire. In doing so, they began making tires with aspect rations lower than the “standard” 82% and so it became necessary to include the AR in the metric tire sizing system


The Flotation Sizing system is used for larger, wider tires (LT tires) used on trucks and SUV’s – so named because these tires are supposed to “float” over soft surfaces. It is a convenient system for us as it gives the whole picture at a glance.



36 = OD in inches
12.50 = Section width (NOT tread width)
R = denotes radial construction (absence of the R denotes bias ply construction)
15 = rim diameter in inches
LT = Light Truck tire (may or may not be present)

Tread width may be published by manufacturer, but for us will vary enormously anyway with load and low air pressures.

LT Numeric

For LT tires there also exists another useful system – the LT numeric system – based on the old Numeric system but with the useful addition of the OD.



9 is the approximate section width (in inches)
34 is the approximate overall tire diameter (in inches)
16 is the rim diameter in inches)
LT designates a Light Truck application


- This system is used only for bias ply tires.


Numeric – old and practically useless (unless you’re just shopping for an exact replacement tire from the same manufacturer for your tractor, small trailer, or wheelbarrow). Tires in these sizes will be bias ply.

Alpha-Numeric – only slightly less useless than the numeric system. Might be of some use to those competing in certain sanctioning bodies with rules based on what is written on the tire. E.g. getting away with an aired-down Q-78/15 in a 35” and under class. May be used for radial and bias ply tires.

Metric – has all we need to know, but calculations must be made. Not common in the sizes hardcore wheelers need (38” OD and up). Used with radial and bias ply tires, indicated by an R,D, or B in the designation. Older metric sizing on bias ply tires used a standard, unstated AR of 82%.

Flotation – Most useful “at a glance” info for us, used with both radial and bias ply tires where a radial will always have an R in the designation but a bias may just have no letter. May or may not include the designation “LT” at the end.

LT-Numeric – Similarly useful as the Flotation system, if a little more complicated to read. Used only for LT tires, and only for bias ply construction.

Both the Flotation and LT-Numeric sizing systems tell us the basics of what we need to know. My preference is for the Flotation system as it tells me the info in an easy to read format in the order that I most care about – OD, then width, then rim diameter. Then again, it might just be because that’s the system I grew up reading (and often dreaming) about!

However, it’s not perfect either – in fact, the metric system does offer one advantage – and that’s “at-a-glance” comparison of the “profile” of different tires. E.g.. If we look at:

235/75/R15 and 235/65/R15

We can immediately see that the latter is a lower profile tire.

Traditionally, we wheelers haven’t cared much about a tire’s profile. We just wanted tall tires, either wide or narrow depending on preference. However, in recent years, with the widespread use of single and double beadlocks allowing low single-digit air pressures (to increase contact patch and therefore available traction) combined with the ever increasing need for performance, stability, and predictability demanded by today’s competitive and recreational rock crawling we have begun to take note of the effect of a tire’s profile on it’s performance.

Interestingly, unlike car guys who use an aspect ratio comparing the tire’s sidewall height to its width – we are more concerned with a tire’s sidewall height compared to its overall diameter. Because very large diameter tires (e.g.. 44”), especially on smaller diameter rims (eg. 15”) give a tire a very tall sidewall, often at very low pressure the sidewall will flex excessively so as well as getting a larger contact patch (which is what we want) we get less lateral stability (hindering the ability to stick a technical line in the rocks), unpredictable handling, and increased damage to sidewall injuries (by driving on the sidewall and/or “sidewall pinch” injuries (where the pressure is so low the sidewall essentially collapses and is pinched between the rim and the rocks)). To avoid these troubles we need to run a little more air pressure – we need to find the sweet spot for our rigs weight and our terrain. Of course, this sweet spot is going to be different for different tire sizes on different rims and different tire constructions (radial vs bias ply) – and comparing these (to arrive at the best tire/wheel/pressure combination for us) is where all current sizing methods fall a little short.

If it were up to me we would introduce a new extreme offroad sizing method that would incorporate all the info in the flotation system, along with the aspect ratio (sidewall to width) as well as a dimension I’ll call "profile ratio". This profile ratio would be similar to the aspect ratio except that instead of comparing the sidewall height to the width, it would compare it to the OD (and therefore take into effect the rim diameters effect on the tires performance). It would be calculated as ((OD - Rim Diameter)/2)/OD.

Where AR is read as “The tire is x% as tall as it is wide” the PR would be read as “the sidewall is x% of the tires OD”.

The system would look like this:



42 = Tires overall diameter mounted on approved rim width, inflated to max psi, unloaded.
15 = Tires section width mounted on approved rim width, inflated to max psi, unloaded.
85 = aspect ratio (sidewall height/width x 100%)
30 = profile ration (sidewall height/OD x 100%)
D = Bias Ply (R=radial, B=Bias belted)
16.5 = rim diameter in inches
LT = Light Truck designation.

Imagine how useful this could be. For example, lets say you want a 42” tire (in my opinion we’re always likely to decide on OD first and foremost – basing that decision on either the largest we can fit or the axles will handle; or on what the comp class rules are). To help you compare the different offerings, lets list all the 42” tires Interco offers in the “Big 4” (the 4 most serious off-road options – i.e. TSL, SX, Bogger, And Irok) using my system and see what it tells us.
Style Current Size BV Size AR PR
Bias Irok 14/42-15LT 42x14(96/32 )D15LT 96 32
14/42-16LT 42x14(93/31 )D16LT 93 31
14/42-16.5LT 42x14(91/30)D16.5LT 91 30
14/42-17LT 42x14(89/30)D17LT 89 30
Bias TSL 15/42-15LT 42x15(90/32)D15LT 90 32
15/42-16LT 42x15(87/31)D16LT 87 31
15/42-16.5LT 42x15(85/30)D16.5LT 85 30
Generic 35x12.5R15LT 35x12.5(80/29)R15LT 80 29

You can see that there are options with Aspect Rations ranging from 80 to 96 and Profile Ratios ranging from 29-32. Now, all we need to do is develop some sort of system for quantifying the relative performance of tires with different AR's and PR's! Not so easy to do! Probably the best we can do is rely on experience and anecdotal evidence, and combine that with a knowledge of different common tire's specs. But it would at least be a place to start.

Load-carrying Capacity

A tire's maximum load is the most weight the tire is designed to carry. Since a tire's load carrying capacity is related to the tire's size and how much inflation pressure is actually used, maximum loads are rated with the tire inflated to an industry assigned inflation pressure.

Additionally, load ranges are used to separate tires that share the same physical size, but differ in strength due to their internal construction. "Higher" load ranges are used to identify tires that have a stronger internal construction, and therefore can hold more air pressure and carry more weight.

Each load range has an assigned air pressure identified in pounds per square inch (psi) at which the tire's maximum load is rated. Listed below are the air pressures at which maximum load is rated for popular P-metric and LT tires:

Standard Load (SL): 35 psi
Extra Load (XL): 41 psi

Light Truck
Load Range C (LRC): 50 psi
Load Range D (LRD): 65 psi
Load Range E (LRE): 80 psi

P-metric tires used on passenger cars are rated to carry 100% of the load indicated on the tire's sidewall. However, if the same P-metric tires are used on light trucks, (pickup trucks and sport utility vehicles for example), their carrying capacity is reduced to 91% of the load indicated on the tire's sidewall. This reduction in load results in causing light truck vehicle manufacturers to select proportionately larger P-metric sized tires for their vehicles to help offset the forces and loads resulting from a light truck's higher center of gravity and increased possibility of being occasionally "overloaded."

For example, P235/75R15 P-metric sized, standard load tires used on cars and light trucks would be rated to carry the following maximum loads at 35 psi:

Cars - Full Value - 2028 lbs.
Light Trucks - 9% Reduced Value - 1845 lbs.

A tire’s load-carrying capacity can be indicated in several different ways. They are:

Load Index – a number between 0 and 279 corresponding to a maximum load in lbs. Most often seen in P-metric and LT-metric radial tires.

Load Rating – a single letter between A and F indicating a tire’s RELATIVE load-carrying capacity i.e. the letter does not correspond to an exact capacity or even range of capacities. All we can tell is that an E rated tire will have a greater capacity (and therefore presumably a more rugged construction / possibly stronger sidewall ) than a C rated tire.

Ply Rating – an even number between 2 and 12, this is an older system, identical to the alphabetic load rating system, and having NOTHING to do with the actual number of plies in a tire’s construction i.e. a 10-ply rated tire could be made from a single ply. This is misunderstood and misused all the time. The key to avoiding confusion is to always make sure you use the words “ply rating” when referring to the load carrying capacity of a tire. Most radial passenger tires have one or two body plies, and light truck tires, even those with heavy duty ratings (10-, 12- or 14-ply rated), actually have only two or three fabric body plies.

Specific Load / Sidewall Imprint – max load and pressure spelled out on the sidewall. E.g. “Max load 1250 lbs at max pressure 30psi cold”. When shopping for tires where actual load capacity is a concern (e.g. for your tow rig) this is the best system to use.

Note that neither the Load Rating nor Ply Rating systems correspond to exact load numbers. In fact, depending on manufacturer, size, construction, single- or dual-configuration, etc. tires with the same load range or ply rating can have widely varying actual load-carrying capacities. For example, from various charts, I have seen the following ranges (note the overlap):

B - 1100-1520 lbs
C - 1765-2205 lbs
D – 1930-3000 lbs
E - 2470-3042 lbs
F – 3415-5205 lbs
G – 3415-6610 lbs
H – 4806-7830 lbs

Rather, the Load rating and Ply rating systems are useful only for relative comparison between tires. For example, all we can really tell is that an E rated tire will have a greater capacity (and therefore presumably a more rugged construction / possibly stronger sidewall ) than a C rated tire.

Here is a chart of common Load Index numbers:

Tire Load Index (number) and Load Capacity (lbs.)
Index Load Index Load Index Load Index Load Index Load
0 99 30 234 60 551 90 1323 120 3086
1 102 31 240 61 567 91 1356 121 3197
2 105 32 247 62 584 92 1389 122 3307
3 107 33 254 63 600 93 1433 123 3417
4 110 34 260 64 617 94 1477 124 3527
5 114 35 267 65 639 95 1521 125 3638
6 117 36 276 66 661 96 1565 126 3748
7 120 37 282 67 677 97 1609 127 3858
8 123 38 291 68 694 98 1653 128 3968
9 128 39 300 69 716 99 1709 129 4079
10 132 40 309 70 739 100 1764 130 4189
11 136 41 320 71 761 101 1819 131 4299
12 139 42 331 72 783 102 1874 132 4409
13 143 43 342 73 805 103 1929 133 4541
14 148 44 353 74 827 104 1984 134 4674
15 152 45 364 75 852 105 2039 135 4806
16 157 46 375 76 882 106 2094 136 4938
17 161 47 386 77 908 107 2149 137 5071
18 165 48 397 78 937 108 2205 138 5203
19 171 49 408 79 963 109 2271 139 5357
20 176 50 419 80 992 110 2337 140 5512
21 182 51 430 81 1019 111 2403 141 5677
22 187 52 441 82 1047 112 2469 142 5842
23 193 53 454 83 1074 113 2535 143 6008
24 198 54 467 84 1102 114 2601 144 6173
25 204 55 481 85 1135 115 2679 145 6393
26 209 56 494 86 1168 116 2756 146 6614
27 215 57 507 87 1201 117 2833 147 6779
28 220 58 520 88 1235 118 2910 148 6944
29 227 59 536 89 1279 119 2998 149 7165
. . . . . . . . 150 7385

Here is a chart of Load Ranges, the corresponding Ply Rating, the approximate equivalent Load Index, approximate load carrying capacity, and nominal maximum inflation pressure. Note that the data is incomplete because these systems were never designed to readily interchange. And again, the figures in italics are APPROXIMATE and can vary widely in different tires.
Load Range Ply Rating

Load Index
Approximate Lbs/tire

A 2 92 1389
B 4 98 1653 35
C 6 104 1984 50
D 8 110 2337 65
E 10 116 2756 80
F 12 122 3307 95
G 14 100-120
H 16 100-120
J 18
L 20 125
M 22
N 24

Speed Rating

Most off-road tires are not speed rated, and most of us have no concern for a tire's speed rating because of the use our tires see. That said – no tire article would be complete without at least a mention of speed ratings – and they might be useful if you’re shopping for tow rig tires.
A tire’s speed rating indicates the maximum, sustained speed at which it can safely operate. Differences are based primarily on how much heat is generated in the tire. The faster you go, the more heat is generated, and heat breaks down the rubber in the tire. A tire’s speed rating will therefore depend on how much heat the tire builds at speed (based on its construction) and how well it can handle that heat (how much damage is done to the tire by the heat). A “Z” rated tire will not build much heat and the heat it will build will not damage it nearly as it would a “J” rated tire.

Speed ratings are usually indicated on the sidewall as a letter code, in combination with the numerical Load Index. This combination of speed and load data is known as the tire’s “Service Description”.



82 = the Load Index is 82 (or 1047 lbs)
S = the tire is rated for sustained operation up to 112 MPH

Here is a chart of Speed ratings:
A1 3 5 J 62 100
A2 6 10 K 68 110
A3 9 15 L 75 120
A4 12 20 M 81 130
A5 16 25 N 87 140
A6 19 30 P 93 150
A7 22 35 Q 99 160
A8 25 40 R 106 170
B 31 50 S 112 180
C 37 60 T 118 190
D 40 65 U 124 200
E 43 70 H 130 210
F 50 80 V 149 240
G 56 90 Z 149+ 240+
W 168 270
Y 186 300
(Y) 186+ 300+

Speed ratings are based on laboratory tests where the tire is pressed against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire's required speed has been met.
It is important to note that speed ratings only apply to tires that have not been damaged, altered, under-inflated or overloaded. Additionally, most tire manufacturers maintain that a tire that has been cut or punctured no longer retains the tire manufacturer's original speed rating, even after being repaired because the tire manufacturer can't control the quality of the repair.

Over the years, tire speed rating symbols have been marked on tires in any of three ways shown in the following examples:
225/50SR16 225/50SR16 89S 225/50R16 89S

Early tires had their speed rating symbol shown "within" the tire size, such as 225/50SR16. Tires using this type of branding were made prior to 1991.

Beginning in 1991, the speed symbol denoting a fixed maximum speed capability of new tires must be shown only in the speed rating portion of the tire's service description, such as 225/50R16 89S.

When Z-speed rated tires were first introduced, they were thought to reflect the highest tire speed rating that would ever be required, in excess of 240 km/h or 149 mph. While Z-speed rated tires are capable of speeds in excess of 149 mph, how far above 149 mph was not identified. That ultimately caused the automotive industry to add W- and Y-speed ratings to identify the tires that meet the needs of new vehicles that have extremely high top-speed capabilities.
W 168 mph - 270 km/h
Y 186 mph - 300 km/h

While a Z-speed rating still often appears in the tire size designation of these tires, such as 225/50ZR16 91W, the Z in the size signifies a maximum speed capability in excess of 149 mph, 240 km/h; the W in the service description indicates the tire's 168 mph, 270 km/h maximum speed.
225/50ZR16 in excess of 149 mph, 240 km/h
205/45ZR17 88W 168 mph, 270 km/h
285/35ZR19 99Y 186 mph, 300 km/h

Most recently, when the Y-speed rating indicated in a service description is enclosed in parentheses, such as 285/35ZR19 (99Y), the top speed of the tire has been tested in excess of 186 mph, 300 km/h indicated by the service description as shown below:
285/35ZR19 99Y 186 mph, 300 km/h
285/35ZR19 (99Y) in excess of 186 mph, 300 km/h

How to Read the Sidewall

There's an awful lot of useful information that is moulded into the sidewall of every tire.
The most useful of which are as follows:

Service Type

Most tire sizes begin with a letter or letters that identify the type of vehicle and/or type of service for which they were designed. The common indicators are as follows:

P225/50R16 91S

P = When a tire size begins with a "P," it signifies the tire is a "P-metric" size that was designed to be fitted on vehicles that are primarily used as passenger vehicles. This includes cars, minivans, sport utility vehicles and light duty pickup trucks (typically 1/4- and 1/2-ton load capacity). The use of P-metric sizes began in the late 1970s and they are the most frequently used type of tire size today.

225/50R16 92S

If there isn't a letter preceding the three-digit numeric portion of a tire size, it signifies the tire is a "Metric" size (also called "Euro-metric" because these sizes originated in Europe). While Metric tire sizes are primarily used on European cars, they are also used on vans and sport utility vehicles. Euro-metric sizes are dimensionally equivalent to P-metric sizes, but typically differ subtly in load carrying capabilities.

T125/90D16 98M

T = If a tire size begins with a "T," it signifies the tire is a "Temporary Spare" ("space saver" or "mini spare") that was designed to be used temporarily only until a flat tire can be repaired or replaced.

LT245/75R16 108/104S

LT = If a tire size begins with "LT," it signifies the tire is a "Light Truck-metric" size that was designed to be used on vehicles that are capable of carrying heavy cargo or towing large trailers. This includes medium and heavy-duty (typically 3/4- and 1-ton load capacity) pickup trucks, sport utility vehicles and full-size vans. Tires branded with the "LT" designation are designed to provide substantial reserve capacity to accept the additional stresses of carrying heavy cargo.

7.50R16LT 112/107Q, 8.75R16.5LT 104/100Q or 31x10.50R15LT 109Q

LT = If a tire ends with "LT," it signifies the tire is an earlier Light Truck size designed to be used on vehicles that are capable of carrying heavy cargo and towing trailers or are wider, oversized tires designed to help the vehicle drive on top of loose dirt or sandy surfaces. This includes light, medium and heavy-duty (typically 1/2-, 3/4 and 1-ton load capacity) pickup trucks and sport utility vehicles. Tires branded with the "LT" at the end of their size designation are designed to provide substantial reserve capacity to accept the additional stresses of carrying heavy cargo.

195/70R15C 104/102R

C = If a Euro-metric sized tire ends with a "C," it signifies the tire is a "Commercial" tire intended to be used on vans or delivery trucks that are capable of carrying heavy loads. In addition to being branded with the "C" in their size, these tires are also branded with their appropriate Service Description and "Load Range" (Load Range B, Load Range C or Load Range D).


ST = If a tire size begins with "ST," it signifies the tire is a "Special Trailer Service" size that was designed to only be used on boat, car or utility trailers. ST-sized tires should never be used on cars, vans or light trucks. Trailer tires have extra strong sidewalls to stand up to the side-side motion of a trailer but not for the loads applied to, or the traction required by, drive or steering axles.

Section Width

Following the letter(s) that identify the type of vehicle and/or type of service for which the tire was designed, the three-digit numeric portion identifies the tire's "Section Width" (cross section) in millimeters.

P225/50R16 91S

The 225 indicates this tire is 225 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel. This measurement is also referred to as the tire's section width. Because many people think of measurements in inches, the 225mm can be converted to inches by dividing the section width in millimeters by 25.4 (the number of millimeters per inch). 225mm / 25.4 = 8.86"

Sidewall Aspect Ratio

Typically following the three digits identifying the tire's Section Width in millimeters is a two-digit number that identifies the tire's profile or aspect ratio.

P225/50R16 91S

The 50 indicates that this tire size's sidewall height (from rim to tread) is 50% of its section width. The measurement is the tire's section height, and also referred to as the tire's series, profile or aspect ratio. The higher the number, the taller the sidewall; the lower the number, the shorter the sidewall. We know that this tire size's section width is 225mm and that its section height is 50% of 225mm. By converting the 225mm to inches (225 / 25.4 = 8.86") and multiplying it by 50% (.50) we calculate that this tire size results in a tire section height of 4.43".

Internal Construction

A letter (R in this case) that identifies the tire's internal construction follows the two digits used to identify the aspect ratio.

P225/50R16, P225/50ZR16

The R in the P225/50R16 91S size identifies that the tire has a radial construction in which the tire's body plies "radiate" out from the centerline of the tire. Radial tires are by far the most popular type of tire today representing over 98% of all tires sold.

If the R in the size was replaced with a D (225/50D16), it would identify that the internal tire body plies crisscross on a Diagonal and that the tire has a bias ply construction.

If the R in the size was replaced with a B (225/50B16), it would identify that the tire body plies not only crisscross the tire on a diagonal, but that they are reinforced with belts under the tread area. This type of tire construction is called "Bias Belted."

Speed Rating

Today, the only tires that continue to include the speed rating "in" the tire size (P225/50ZR16) are Z-speed rated tires. In this case, following the two digits used to identify the aspect ratio are the letters ZR to identify the tire's speed rating (Z) and its internal construction (R). Since 1991, all other speed ratings are identified in the tire's Service Description (see below).

Tire and Wheel Diameter

P225/50R16 91S

The 16 indicates the tire and wheel diameter designed to be matched together.

Tires that have a rim diameter expressed in inches (P225/50R16, as well as 8, 10, 12, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26 and 28) are called "inch rim" sizes, are the most common type of tire size and are used on most cars, minivans, vans, sport utility vehicles and light duty light trucks.

While not as common, two additional "unique" types of tire/wheel diameters are still in use today.

Tires and wheels that have a rim diameter expressed in "half" inches (8.00R16.5LT, as well as, 14.5, 15.5, 17.5 and 19.5) are used on some heavy-duty trailers, heavy-duty light trucks and box vans.

Tires and wheels that have a rim diameter expressed in millimeters (190/65R390, as well as, 365 and 415) are called millimetric sizes. Michelin initiated millimetric sizes for their TRX tires that saw limited use on many different car models in the late 1970s and 1980s.
Michelin PAX System run flat tires have been introduced as an integrated wheel/tire system on a very limited basis as Original Equipment (O.E.) in North America. An example PAX System size of 235/710R460A 104T expresses tire and wheel dimensions in millimeters (235 mm Section Width, tire Overall Diameter of 710 mm and a 460A mm rim diameter, with the "A" in 460A signifying these tires feature “asymmetric” beads in which the outside bead (450 mm) and inside bead (470 mm) are actually different diameters.

All of these "unique" tire/wheel diameters were developed specifically because the tire and wheel design or intended vehicle use required them to be different than conventional tires and wheels. All of these tires and wheels feature bead profiles that have a different shape than traditional "inch rim" sizes.

Tires and wheels with unique rim diameters should never be combined with traditional "inch rim" tires and wheels.
It is critical that the tire and wheel diameters are always confirmed to match before the tire is mounted on the wheel.

Service Description

P225/50R16 91S

The 91S represents the tire's Service Description. A Service Description identifies the tire's Load Index and Speed Rating. Service Descriptions are required on all speed rated (except for Z-speed rated) tires manufactured since 1991. For more information on Load Index and Speed Rating see he individual sections in this article.

Load Range

LT245/75R-16E, 7.50R-15D, 31x10.50R-15C, ST205/75R15 LRC

If a tire does not use the Service Description on the sidewall, it’s load range will be indicated as part of the tire’s description, as shown in the above examples, using the Load Rating letter designation system described earlier in this article.


LT255/70R16 XL M+S

XL or RF designates an Extra Load or Reinforced tire. If not specified, Standard Load (SL) is assumed and will usually not be labeled as such.

M+S designates a tire rated for mud and snow use.

"MAX LOAD SINGLE 2623 lbs. at 65 psi COLD"

This indicates the maximum load rating of the tire and corresponding minimum cold inflation pressure.
Note that P and LT tires are often rated slightly different. If P-rated tire is used in an LT application, you need to de-rate the sidewall load capacity by 9% - 10% according to many tire makers.

The branding on the sidewall of a tire is required to list the materials and number of layers of each material used to reinforce the rubber.
A typical tire's basic construction materials are usually presented as follows:


The branding in this example identifies that molded into the rubber under the centerline of the tread lies two radial body plies of polyester cord, two belts of angled steel cord and one circumferential cap ply of nylon cord. It also identifies that in each sidewall at the widest points between the tire's inner and outer sidewalls (tire section width) lie two radial body plies of polyester cord (a continuation of the same two body plies that were listed under the centerline of the tread).

Tire FAQ:

What’s the best way to replace a single tire or pair of tires on a vehicle?

Never mix radial and bias ply tires on the same axle (because of their different handling characteristics discussed earlier in this article)
If you have radials on one axle and bias ply on another, run the radials on the rear
If you are replacing a pair of tires, put the new ones on the rear
If you have to replace just one tire, pair the new one with the best of the other three and run them on the rear

What is the maximum weight it should take to balance my tire?

The industry standard is approx. 2% of the tire weight. So, because a 42” TSL weighs about 100lbs, 2% is 2lbs or 32ozs of lead! That’s industry standard. Keep this in mind next time you’re wondering why your 42” Swampers don’t balance well - it’s not because of the quality of the tire – it’s because it weighs so much – it’s just the physics of dynamically balancing that much rolling mass – and it’s the price you must pay for large, aggressive tires.

What are tires made of?

The average steel-belted radial tire is made up of:

28% Carbon Black
27% Synthetic Rubber
14% Natural Rubber
10% Steel Wire
10% Extender Oil
4% Organic Fiber
4% Other Petroleum Products
3% (S, ZnO, Ti02, etc...)

How round is round?

Generally, a passenger-car tire is considered round if it has less than 0.030" radial or lateral runout. An LT tire is considered round if it has less than 0.060" radial and lateral runout.

Does it matter how I mount the tire on the rim?

There are two schools of thought on this.

The first has that you should mount the high-spot of the tire (often indicated by a red dot) aligned with the low-spot of the rim (often the valve hole, but may be separately indicated, depending on the wheel in question).

The other method is to mount the lightest spot of the tire (often indicated by a yellow dot) aligned with the heaviest spot on the rim (the valve).

What effect do different rim widths have on my tires?

The wider the rim, the greater the section width. Section width increases about 0.4” for each additional inch of rim width (and vice versa).

A wider rim increases the distance between the beads, which results in a straighter sidewall, which stiffens it. The straighter sidewall also exposes the rim, making the wheel more susceptible to damage.

A narrower rim pulls the beads closer together, curving the sidewalls. This increased curvature allows the sidewall to flex more readily. It can also help with bead retention at lower air pressures, although using narrow rims is no substitute for beadlocks.

Why do race teams use Nitrogen in their tires instead of air?

Race cars, aircraft, and other ultra-high performance machines use Nitrogen in their tires instead of air because, when heated or cooled, nitrogen has a much more consistent rate of expansion and contraction than air.
This is because air contains varying amounts of moisture due to changes in the relative humidity. This water vapour causes air to be inconsistent in its rate of expansion and contraction.

What is the difference between an LT and P tire of the same size?

LT tires that are load range C or greater tend to be rougher riding and noisier than their P-Metric counterparts and more expensive since the LTR size equivalent to the P-Metric has heavier body ply construction and the tread lugs are generally more aggressive.

P and LT tires are also often rated slightly differently. If P-rated tire is used in an LT application, you need to de-rate the sidewall load capacity by 9% - 10%.

How should I store unmounted tires?

Store them on their sides in a clean, cool, dry, dark, and well-ventilated area (but with a minimum of circulating air).
Keep them out of direct sunlight.
Keep them away from sources of ozone such as operating welders and electric motors. Store tires away from the furnace, sump pump, power tools, etc.
If you must stack them, do so only to a height where the bottom tire retains its shape, usually no more than four high.

What about tires on my rig in storage?

Store the vehicle on blocks to remove all weight from the tires.
If the vehicle cannot be blocked up, completely unload it so minimum weight will rest on the tires. The surface should be firm, reasonably level, well drained, and clean.
Keep tires inflated to recommended operating pressure.
Move the vehicle at least every three months to prevent ozone cracking in the bulge area, to help maintain oil dispersion within the rubber compounds, and to prevent a "flat spot" from developing.

What’s so special about “ST” tires for my trailer? Why can’t I replace them with P tires with an equivalent load rating?

“ST” or “Special Trailer” tires are designed specifically for trailer service, whether they are radial or bias ply. They feature materials and construction to meet the higher load requirements and unique demands of trailering. For example, ST tires are constructed for better high speed durability and bruise resistance under heavy loads and they have stiffer sidewalls that help to reduce trailer sway. Passenger car (P) or light truck (LT) tires do not have the structural components to provide stability and handle the stress and dynamics imposed by a trailered load.

ST tires typically have larger chords in the body plies and the steel wire in the beads has a larger diameter and greater tensile strength to meet the additional load requirements. ST tire rubber compounds also contain more chemicals to resist weather and ozone cracking.

Similarly, trailer tires are designed for use on trailer axle positions only, not for the loads applied to, or the traction required by, drive or steering axles.

Are there special considerations with trailer tires that are not present with other tires?

While the majority of the following factors also apply to other tires, they have special significance for trailer tires. There are two main reasons for this. First, absolutely nothing good happens when you experience a tire failure while towing a heavy load. A blowout in your car during the morning commute is bad enough – but when it happens as you’re towing 4 tons at 70mph in the dark on a curvy, rain-slick road the stakes are incredibly high! Secondly, trailer tires tend to get used, abused, and neglected or forgotten about. Because we don’t use them every day and because there’s nothing sexy about them we tend to take them for granted and not pay them the attention they deserve. We do so at our own peril!

· Always inflate trailer tires to the maximum inflation indicated on the sidewall.
· Check inflation when the tires are cool and have not been exposed to the sun.
· If the tires are hot to the touch from operation, add 3 psi to the max inflation.
· Underinflation is the number 1 cause of trailer tire failure.
· Tires lose approximately 1 psi per month as well as 1 psi for every 10-degree drop in temperature.

Load Carrying Capacity
· All tires must be identical in size for the tires to properly manage the weight of the trailer.
· The combined capacity of the tires must equal or exceed the GVW of the axle.
· The combined capacity of all of the tires should exceed the loaded trailer weight by 20 percent. If the actual weight is not available, use the trailer GVW
· If a tire fails on a tandem axle trailer, you should replace both tires on that side. The remaining tire was likely subjected to excessive loading.
· If tires are replaced with tires of larger diameter, the tongue height may need to be adjusted to maintain proper weight distribution.
· One key to extending tire life on a tandem- or tri-axle trailer is to ensure that the trailer is riding level, thus distributing the load equally among all the tires. If the trailer tongue sits too high, the rear tires may bear the brunt of the load: with the trailer tongue too low, the front tires may be unduly stressed.

· All "ST" tires have a maximum speed rating of 65 mph.
· As heat builds up, the tire's structure starts to disintegrate and weaken.
· The load carrying capacity gradually decreases as the heat and stresses generated by higher speed increase.

· Time and the elements weaken a trailer tire. The structural components and bonding agents slowly break down. This is due primarily to internal air pressure forcing oxidation of the tire materials. Ultraviolet rays also attack the rubber on a tire left exposed to the sun. As a result, a 15-year-old tire that was rarely used may look virtually new, but because of the ravages of time and elements, it does not have the same strength as when it was new.
· In about 3 years roughly one third of the tire's strength is gone.
· Three to five years is the projected life of a normal trailer tire.
· It is suggested that trailer tires be replaced after 3 to 4 years of service regardless of tread depth or tire appearance.

· Trailer tires are not designed to wear out.
· The life of a trailer tire is limited by time and duty cycle.
· The mileage expectation of a trailer tire would be 5,000 to 12,000 miles.

· The ideal storage is in a cool, dark garage at maximum inflation.
· Use tire covers to protect the tires from direct sunlight.
· Use thin plywood sections between the tire and the pavement.
· For long term storage: Put the trailer on blocks to take the weight off the tires, lower the air pressure and cover tires to protect from direct sunlight.

Glossary of Tire Terms:

Air Pressure:
The amount of air inside the tire pressing outward on each square inch of tire; expressed in pounds per square inch (psi).

A term for describing the size of a tire in which both letters of the alphabet and numbers are used.

A synthetic fabric used in some tires that is (pound-for-pound) stronger than steel.

Aspect Ratio:
The relationship of a tire's height to its width, expressed as a percent (%).

A round hoop of steel wires, wrapped or reinforced by ply cords, that is shaped to fit the rim; holds the tire onto the rim.

Belted Bias Tire:
A pneumatic tire with a body similar to that of bias tires, but which also includes two or more belts under the tread.

A special rubber-coated layer (or ply) of cords running circumferentially around the tire, located between the body plies and the tread. The cords may be made from steel, fiberglass, rayon, nylon, Kevlar, polyester or other fabrics.
They are designed to reinforce body plies to hold the tread flat on the road. Belts reduce squirm to improve tread wear and resist damage from impacts and penetration.

Bias Ply Tire:
A pneumatic tire in which the plies are laid at alternate angles less than 90 degrees to the center line of the tread. Plies usually run at angles about 30-40 degrees to the center line in a crisscross fashion.

AKA Casing. The main body of the tire, beneath the tread and sidewalls, consisting of wire beads and body plies. It forms the foundation for the tread and sidewalls.

Cold Inflation Pressure:
The amount of air pressure in a tire, measured in pounds per square inch (psi) before a tire has built up heat from driving.

The general term referring to the chemical formula for the tread material.

The strands of fabric forming the plies or layers of the tire. Cords may be made from fiberglass, rayon, nylon, polyester, Kevlar or steel.

DOT Markings:
A code molded into the sidewall of a tire signifying that the tire complies with U.S. Department of Transportation motor vehicle safety standards. The DOT code includes an alphanumeric designator which can also identify the tire's manufacturer, production plant, date of production and brand.

AKA Contact Patch. That portion of the tread that contacts the road.

The space between two adjacent tread ribs; also called tread grooves.

Inner liner:
A layer of specially compounded rubber forming the innermost layer of a tubeless tire, designed to inhibit loss of air pressure.

Loaded section height:
The height of the section of the tire that is making contact with the road.

Load Index:
An assigned number ranging from 0 to 279 that corresponds to the load carrying capacity of a tire.

Maximum Inflation Pressure:
The maximum air pressure to which a cold tire may be inflated; found molded onto the sidewall.

Nominal rim diameter:
The diameter of a tire rim.

Overall diameter:
The diameter of the inflated tire, mounted on the approved width rim, without any load.

Overall width:
The distance between the outside of the two sidewalls, including lettering and designs.

Uniform designation of tire sizes, in metric measurements originally introduced by American tire manufacturers in 1977; commonly called "P-metric series." A typical P-metric tire is P205/70R14 93S.

A layer of heat and impact resistant rubber-coated fabric containing cords that run parallel to each other; extends from bead to bead and goes between the inner liner and belts or tread; used to form the body of the tire. Automobile and light truck tire plies are normally constructed of nylon or polyester cords.

Radial Tire:
A type of tire with plies arranged so cords in the body run at 90 degree angles to the center line of the tread.

A metal support for a tubeless tire or a tire and tube assembly upon which the tire beads are seated.

Rim width:
Distance between the two opposite inside edges of the rim flanges.

Section height:
The distance from rim seat to outer tread surface of an unloaded tire.

Section width:
The linear distance between the outside sidewalls of an inflated tire without any load (exclusive of protruding side ribs, raised lettering and decorations).

A numerical representation of a tire's aspect ratio; for example, 50 series Tires with the same aspect ratio, or relationship of height to width, are said to be the same “series”.

The area of a tire where the tread and sidewall meet.

Shoulder Blocks:
Raised rubber compound segments on the part of the tire tread nearest the sidewall.

That portion of a tire between the tread and the bead.

Special slits within a tread that increase wet and snow traction.

The process of introducing sipes (or slits) into the tread.

The combination of tire width, construction type, aspect ratio and rim size used in differentiating tires.

Speed Rating:
An alphabetical code (A-Z) assigned to a tire indicating the range of speeds at which the tire can carry a load under specified service conditions.

Steel Belt:
A belt material used in radial tires. Its high stiffness provides good handling and low tread wear.

A precisely engineered assembly of rubber, chemicals, fabric and metal designed to provide traction, cushion road shock and carry a load under varying conditions.

Tire Designation:
An alphanumeric code molded into the sidewall of the tire that describes the tire's size, including width, aspect ratio, rim diameter, load index and speed rating. Most designations use the P-Metric system.

That portion of a tire that comes into contact with the road. It is distinguished by the design of its ribs and grooves.

Tread Blocks:
Raised rubber compound segments on the outside visible part of a tire.

Tread Rib:
The tread section that runs around the circumference of the tire, separated by the tread grooves.

Tread wear Indicator:
Narrow bands, sometimes called "wear bars," that appear across the tread of the tire when only 2/32 inch of tread remains.

Tread width:
The width of a tire's tread that comes in contact with the road.

The friction between the tires and the road surface; the amount of grip provided.

(Uniform Tire Quality Grading Standards) A tire information system that provides consumers with ratings (from A to C) for a tire's traction and temperature. Tread wear is normally rated from 60 to 620. Ratings are determined by tire manufacturers using government-prescribed test procedures, and are moulded into the sidewall of the tire.

A device that lets air in or out of a tire. Fitted with a valve cap to keep out dirt and moisture and a valve core to prevent air from escaping.

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InläggPostat: lör aug 27, 2011 11:48 

Blev medlem: mån mar 21, 2011 18:06
Inlägg: 79
Ort: Gräsmyr / Umeå
Detta kommer till största del från Pirate4x4 av "BillaVista".
Var bara tvungen att lägga hit detta då jag tidigare sökt en sånhär sammanställning utan resultat och nu hittade jag detta av en slump.

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InläggPostat: lör aug 27, 2011 18:06 

Blev medlem: tis sep 22, 2009 22:19
Inlägg: 506
Ort: Umeå

Tråden borde låsas

Daniel Sjöström
Nissan patrol 2,8td -91
Bmc mini 1000 -68

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InläggPostat: tis feb 21, 2012 18:21 

Blev medlem: tis okt 14, 2008 16:24
Inlägg: 556
Ort: CheeseCoast
Ja bara nåla på men den behövs väl inte låsas? ;)


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InläggPostat: sön okt 14, 2012 12:00 

Blev medlem: mån aug 03, 2009 17:15
Inlägg: 262
Ort: värmland
http://www.tuffcountry.com/customer-car ... guide.html


K1500 silverado -99 SÅLD

Mitsubishi L200 DC 2,5 TD -04

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InläggPostat: fre jun 07, 2013 3:36 

Blev medlem: lör maj 18, 2013 15:33
Inlägg: 398
Ort: Gävle
Jag fattar inget av det hela.
Finns det någon enklare förklaring eller kanske något som kan göra det automatiskt.
Visst hade jag tur att det däck dom använder är det som sitter orginal.
Therefore a 235/75/R15 is a 29” x 9.25” R15

Men på bilen idag sitter 31x12,5x15 och jag lyckas inte få fram mm måttet...

Volkswagen Passat -98 med Syncro. *Brukis*

GMC Jimmy -87 på 33" däck. *Leksak*
Garaget tråd

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InläggPostat: ons jun 12, 2013 6:52 

Blev medlem: mån mar 19, 2012 16:14
Inlägg: 193
Ort: Södra Härene, Vårgårda

Obotlig anglofil som aldrig blir vuxen.
Land Rover Discovery ll TD5
Land Rover Discovery l Trophy TDI
Land Rover Discovery l V8i
Range Rover Classic MB300TDI

övrigt i garaget:
Jaguar XKR
Jaguar Sovereign 3,6
Triumph 2000 mk1
Triumph 2000 mk1

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InläggPostat: tis apr 15, 2014 18:18 

Blev medlem: mån okt 06, 2008 18:43
Inlägg: 156
Ort: Rättvik
Jag gjorde också en liten smidig däckkalkylator för som automatiskt konverterar mellan tum och metriska däckdimensioner på min hemsida som ni gärna får testa:

http://www.4x4expedition.se - Expeditioner där DU kan delta!
http://www.4x4offroad.se - Allt du behöver för din offroad- och expeditionsbil!

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InläggPostat: sön maj 31, 2015 18:04 

Blev medlem: tis aug 26, 2008 17:51
Inlägg: 757
Ort: Vetlanda
Jag har tittat en del på superswamper irok. Däcket finns i några varianter jag inte förstår, radial, diagonal och bias. Kan någon förklara skillnaden på dom olika varianterna så man förstår :)

Willys cj2a
Volvo L2304
kingquad 750LE

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InläggPostat: sön maj 31, 2015 19:56 

Blev medlem: mån nov 05, 2007 20:19
Inlägg: 1061
Ort: Vetlanda
svahnström skrev:
Jag har tittat en del på superswamper irok. Däcket finns i några varianter jag inte förstår, radial, diagonal och bias. Kan någon förklara skillnaden på dom olika varianterna så man förstår :)

Radial och diagonal(bais) är hur koden på däcket är uppbyggt. Ett vanligt bildäck för vägbruk är radialdäck medans skogsmaskinsdäck oftast är diagonaldäck. Diagonal(bais) däck är mer sidostabila och klarar mer last.

Fan, jävla oflyt hela ti'n!

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