Tyre Coding

The sidewall of a typical tyre is imprinted with myriad codes and numbers that are used to identify the size and physical features of that tyre as well as details relating to its manufacture.

Tyre Size

Your tyre size can be found on the sidewall of your current tyre and is a sequence of numbers and letters.

• The first three digits displays the width of the tyre in millimetres. This measure is across the tread from sidewall to sidewall.
• The fourth and fifth digits of the tyre code that immediately follow the tyre width is the aspect ratio or profile height of the tyre sidewall. It is expressed as a percentage of the tyre width.
• Radial tyres are marked with the letter R. Radial tyres are constructed with the cord plies positioned at a 90 degree angle to the direction of travel to give the tyre additional strength. Almost every new tyre manufactured today is a radial tyre.
• The next two digits represent the size of the wheel rim that the tyre can be fitted to. It is also the diameter of the tyre from bead to bead.

Speed Rating

The speed rating of a tyre is represented by a letter of the alphabet at the end of the tyre size code and indicates the maximum speed capability of the tyre. Tyres receive a speed rating based on a series of tests which measure the tyres capability to handle a set speed for a prolonged period of time.

Load Index

The load index provides information on the maximum weight capability for the tyre. The load index is a numerical code that can be located just after the tyre size marking and before the speed rating.

Date of Manufacture

Your tyre is also printed with information about when and where the tyre was manufactured. This is represented by a series of letters and numbers which starts with the letters DOT (meaning the tyre exceeds Department of Transport safety standards).

• The first 8 characters that follow DOT are essentially a serial number used by the manufacturer to identify the tyre and in which production facility it was made.
• The last four digits, usually presented in a small window can help you to identify the age of the tyre.
• The first two digits represent the week of the year the tyre was produced from 1 to 52. The last two digits represent the year. 

Reinforced

Tyres with an increased load index are often marked to show that they have been reinforced to handle extra weight. Common markings to show this are: RF, REINF and RFD.

Run Flat

Run flat tyres are specially designed with strengthened side walls so that, in the event of a loss of pressure such as a puncture, the tyre can hold the weight of the vehicle and continue to be used for a short time until you can reach an appropriate place to stop and have the tyre replaced. Possible symbols for this are: ROF, EMT, RFT, ZP, SSR and DSST.

Homologation

Some tyres are intended specifically for use with certain vehicle makes for added performance benefits. These tyres are designed in close co-ordination with the vehicle manufacturer and feature a designated symbol on the sidewall which shows that the tyre meets the manufacturer’s exacting standards and has been given OE status. Tyres designed for Ford vehicles will have an FO symbol.