How To Understand the tyre side-wall data

The majority of motorists understand that tread depth is an important indication of a tyre's lifespan and are aware that tyre pressures need to be maintained; however, a surprising number of people are unaware of the sheer amount of information that a tyre carries on its sidewall.

The most popular tyre size in this country is 205/55R16. The first three numbers - 205 - refer to the tyre's width in millimetres. The next two numbers after the backslash - 55 - represent the aspect ratio of the sidewall, which in this instance means that the height of the tyre is 55 per cent of the width.

The 'R' represents radial, which applies to most modern tyres, and the final digits - 16 - indicate the size of wheel rim that the tyre will fit.

The next two digits represent the load index, or maximum weight capability, of the tyre. There is no legal requirement to keep to these figures or to match the load index for all tyres on a vehicle; however, it is certainly recommended that you adhere to the guidelines set out by your car manufacturer, as laid out in your vehicle handbook.

The letter that follows the load index indicates the maximum speed at which the tyre should operate. 'Q' is the lowest, representing a speed of 99mph, with 'Y'-registered tyres travelling at speeds of up to 186mph. It is important to keep to the speed suggested by your car's manufacturer; otherwise, you risk invalidating your insurance.

Next, you will see a series of numbers and letters preceded by the letters DOT. This shows that the tyre's design and manufacture exceeds the safety standards set out by the Department of Transport. The next eight characters are the manufacturer's identification code, followed by four digits - often displayed within a box - that show the date of manufacture: the first two show the week, from 01-52, followed by two digits to represent the year. Most manufacturers recommend changing tyres at least every six years, even if they still appear to have sufficient tread.

Any remaining characters indicate whether the tyre benefits from some form of run-flat technology, whereby reinforced sidewalls give you the benefit of a few extra miles in the event of a puncture. Reinforced tyres can hold greater weight than their load index suggests, with different tyre manufacturers having their own codes.

Any further characters indicate whether the tyre has been developed in association with a specific car manufacturer and is therefore recommended for specific models.

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About the author

David Sholicar

David is the National Retail Operations Manager for Protyre. One of David’s areas of responsibility and ...

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