Tire Size Explained: What Do All the Numbers Mean?

Dec 10, 2018 | Guides

Tire specifications are detailed 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 Index Values
Load Index Carrying Capacity (lbs)
70 739
71 761
72 783
73 805
74 827
75 852
76 882
77 908
78 937
79 963
80 992
81 1019
82 1047
83 1074
84 1102
85 1135
86 1168
87 1201
88 1235
89 1279
90 1323
91 1356
92 1389
93 1433
94 1477
95 1521
96 1565
97 1609
98 1653
99 1709
100 1764
101 1819
102 1874
103 1929
104 1984
105 2039
106 2094
107 2149
108 2205
109 2271
110 2337
111 2403
112 2469
113 2535
114 2601
115 2679
116 2756
117 2833
118 2910
119 2998
120 3086
121 3197
122 3307
123 3417
124 3527
125 3638
126 3748

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 Index Rating
Speed Rating Miles/Hour
M 81 mph
N 87 mph
P 93 mph
Q 99 mph
R 106 mph
S 112 mph
T 118 mph
U 124 mph
H 130 mph
V 149 mph
W 168 mph
Y 186 mph
Z 149+ 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.

Richard Reed

Writer for TheVehicleLab.com

The Vehicle Lab

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