Tire Size Explained: What Do All the Numbers Mean?

example tire sizing

Tire size specifications are outlined by the numbers and letters found on the sidewall of tires.

It is important that we understand tire geometry and be informed of the parameters that govern their performance. Tire sizes are defined by specifications such as the type, width, aspect ratio, speed rating, tire construction type, and service description.

There are two organizations, Tire and Rim Association (TRA), and European Tyre and Rim Technical Organisation (ETRTO), that influence tire standards.

Example of Tire Size Specifications

The following is an example of tire specifications. We will be using it as an example throughout

P 185 / 60 R14 82H

Tire Type

Tire sizes will often start with letters. For example P, LT, T, and ST.

P

When we use tire type “P” or P-Metric, it means that the tire is made according to certain standards of the United States (drafted by TRA) for passenger vehicles.

If there are no letters at the beginning, it refers to the Euro-metric (drafted by ETRTO). The P-metric and Euro-metric differ in load capacities.

LT

LT or Light Truck are tires for towing trailers or have a 3/4 to 1 ton load capacity.

T

A tire labeled “T” or Temporary Spare is meant to be used in the event of a flat tire.

ST

Tires labeled “ST” or Special Trailer are meant for trailers. Including but not limited to boat trailers, utility trailers, and 5th wheels. ST tires should never be used on cars.

Tire Width

The tire width is measured from sidewall to sidewall of the tire. It is represented by a three digit number and measured in millimeters.

In our example above, the width is 185mm.

Aspect Ratio

This is the ratio of the height of the tire’s cross-section to its width. For example, the number 60 in the above illustration represents that the height is equal to 60% of the width.

This factor shows how sensitive the tire’s reaction to the texture of the road is. The bigger the aspect ratio, the thicker the sidewall of the tire. Heavy vehicles like buses and trucks have a larger aspect ratio, which may compromise the handling.

However, they provide a more comfortable ride, as there is more air in the tires, which can cushion things like bumps.

Construction

This letter denotes the style of the tire. The letter “R” in the above example shows the tire construction type as ‘radial.’

R

R or “Radial” tires are the most popular style of tire out there. In this setup, in the carcass, the cable plies radiate around the axis of the tire. The crown consists of plies that form a belt.

Typically radial tires are going to have a better grip on the road which results in a more comfortable ride. These tires will also wear evenly since the point of contact is relatively uniform throughout.

B

B or “Bias” feature a carcass that consists of diagonally oriented cable plies. This is often referred to as crossply.

The footprint of this tire is longer in comparison to the radial tire, but it is also skinnier, which provides less grip on the road and offers a more rigid ride. At high speeds, bias tires can also deform which can affect handling.

Wheel Diameter

The size of the wheel from one end of the rim to the other is the wheel diameter. It is denoted in inches.

Since the wheel mounts the tire, when we need to use a spare tire, the wheel diameter parameter will help us decide if the spare/replacement will fit on the wheel.

Load Index

The load index is the maximum load that the tire can support. The bigger the value, the more load that it can bear when properly inflated. These load indexes correspond to carrying capacities. For instance, in our example, “82” is 1047 lbs.

Note that Light truck tires have two load indexes, unlike passenger tires, because LT tires are mostly used on vehicles with dual rear wheels. The first load index marks the load capacity when used as a single tire. The second index marks the load capacity when used for two tires.

Tires typically range from 70-126 for their load index. You can expand the table below to find your load index rating and the corresponding carrying capacity.

Load IndexCarrying Capacity (lbs)
70739
71761
72783
73805
74827
75852
76882
77908
78937
79963
80992
811019
821047
831074
841102
851135
861168
871201
881235
891279
901323
911356
921389
931433
941477
951521
961565
971609
981653
991709
1001764
1011819
1021874
1031929
1041984
1052039
1062094
1072149
1082205
1092271
1102337
1112403
1122469
1132535
1142601
1152679
1162756
1172833
1182910
1192998
1203086
1213197
1223307
1233417
1243527
1253638
1263748

Speed Index

This rating indicates the ability of the tire to handle a maximum speed corresponding to the maximum load, without suffering any deterioration in performance.

For example, an H rating has a maximum speed capability of 130 mph or 210 km/hr. On the other hand, a ‘Z’ rated tire has no maximum speed. This rating is for more than 149 mph. This is why sports cars mostly use the Z rating.

Speed RatingMiles/Hour
M81 mph
N87 mph
P93 mph
Q99 mph
R106 mph
S112 mph
T118 mph
U124 mph
H130 mph
V149 mph
W168 mph
Y186 mph
Z149+ mph

Additional Parameters

There could be additional parameters for 2 or 3 wheelers, buses, agricultural vehicles, etc. These indicate different purposes. For instance, the M/C indicates that the tire is for motorcycles only.

P 185 / 60 R14 M/C 82H

In addition to the tire size, there are other parameters on the tire sidewall as well. A few of them are:

  • The brand name
  • Tire performance specification criteria
  • Winter tire symbol
  • UTQG ratings
  • DOT tire identification number
  • Maximum cold inflation load limit

As well as others.

References

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