Vehicle tyres are described by an alphanumeric tyre code, which is moulded into the sidewall of the tyre. This code specifies the dimensions of the tyre, and some of its key limitations, such as load-bearing ability, and maximum speed. Sometimes one sidewall contains information not included on the other sidewall, and vice versa.

The code has grown in complexity over the years, as is evident from the mix of SI and imperial units, and ad-hoc extensions to lettering and numbering schemes. New automotive tyres frequently have ratings for traction, treadwear, and temperature resistance (collectively known as "The Uniform Tyre Quality Grade" (UTQG) ratings.)

Most tyre sizes are given using the ISO Metric sizing system. However, some pickup trucks and SUVs use the Light Truck Numeric or Light Truck High Flotation system.

Explanation of tyre size codes

Tyre size identification diagram

The ISO Metric tyre code consists of a string of letters and numbers, as follows: -
  • Width

Three digit number: The "nominal section width" of the tyre in millimetres; the widest point from both outer edges (side wall to side wall). The tyre surface that touches the road usually has smaller width.
  • Slash (/)

Character for character separation.

  • Aspect ratio

Also referred to as "profile", it is two digit number that represents the sidewall height. It is expressed as a percentage of the nominal section width of the tyre. If aspect ratio is omitted, it is assumed to be 82% (if written, it should be like xxx/82).

  • Construction

A letter or dash indicating construction of the carcass of the tyre:
  • (Dash): bias belt (where the sidewalls plies are the same as the tread, leading to a rigid ride).
  • R: means that the tyre has radial ply construction, meaning the way in which it has been constructed. Modern car tyres are constructed this way so you will rarely find a car tyre without an R. Radial allows the sidewall to have lesser plies hence a much comfortable ride.
  • Wheel diameter

One or two digit number: Diameter in inches of the wheel that the tyres can be assembled with. The number may be longer where a half-inch size is used, for example many heavy transport trucks now use 22.5 inch tyres.

  • Load Index

2- or 3-digit number: Load index. Some light-truck tyres are designed for "dual use", that is they can be run in pairs next to each other. If so, separate load indexes will be specified for single and dual usage. In the example shown in the light-truck tire illustration, the tire has a load index of 114 if used as a single tire, and a load index of 111 if used in a dual pair Tires without this designation are unsafe for dual usage.

  • Speed Index

A letter indicating the speed rating of the tyre. Alternatively, the letter may appear at the end, following the load index. The letter here is Z, indicating a maximum speed in excess of 240km/h (149mph), then a more specific letter W or Y may appear after the load index (see speed rating, below).
    • or 2-digit/letter combo: Speed rating; see table below
  1. Additional marks.
References
  1. wikipedia.org