Learn all there is to know about tyre markings with our helpful tyre markings guide. Found on the tyre sidewall, these letters and numbers refer to the width, height, diameter, load and speed parameters of your tyres.
Understanding these markings will not just improve your car knowledge, but will also help you make the right decision when your old tyres need replacing. Read our guide from etyres.co.uk below.
Tyre width is measured in millimetres (mm) and for the vast majority of car tyres always ends with the number 5.
Tyre width is important as it can affect both your car’s performance and how economical it is with fuel. The amount of rubber that is in contact with the road (called the contact patch) impacts your tyres grip and rolling resistance. The larger the contact patch, the greater the grip but the higher the rolling resistance, resulting in lower fuel economy.
Section height is the height of the tyre measured in millimetres (mm) from the lowest point of the bead (this is what anchors your tyre to the rim) to the highest point of the tread.
When reading your tyre this is displayed as “profile” which is the section height as a percentage of the section width.
Low profile tyres were originally fitted to higher performance cars as they offered more grip during high speed cornering. In recent years however low profile tyres are more often fitted to cars for aesthetics rather than their performance benefit.
Rim diameter is the size of the metal wheel rim that fits in the centre of the tyre. The majority of car wheels are measured in whole inches and range from 12” to 22”.
Larger diameter tyres are preferable for high performance vehicles as the tread has less curvature, creating a larger contact patch and increased grip. Like low profile tyres, large diameter tyres and rims are now fitted onto most modern cars more for aesthetic than performance reasons.
Using the “tyre load index table” which can be found online, your tyre’s load index indicates the maximum weight that your tyre can safely support. This way of measuring a tyre’s weight capacity is relatively modern and was introduced to provide a more consistent method of comparing tyres.
Previously a tyre’s strength related to the number of layers of nylon fabric and steel reinforcement (ply) that were woven into the tyre during manufacture which wasn’t always consistent across different manufacturers.
Speed index is a letter which relates to the table on the left. It represents the maximum speed that your tyre can safely run for a maximum of 10 minutes while supporting the maximum weight indicated by its load index. Every vehicle has a minimum required speed index as required by the manufacturer.
It is acceptable, legal and safe to fit a higher speed index tyre than originally fitted, but it is not recommended to fit a lower speed index as this may invalidate your insurance.
Winter tyres are an exception and it is permitted by most vehicle manufacturers to reduce the speed index fitted by one step.