Tyres Explained: What do the numbers on tyres mean?

Author Name – Louise Helsby

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The sidewall of a tyre will contain information including the size, width and diameter of the tyres required for your vehicle. Here is our comprehensive guide to understanding how to read the tyre size from the sidewall.

What do the numbers and letters on the sidewall mean?

There are numerous letters and numbers printed on the side of a tyre, most of which are of little importance to drivers, however, if you are looking to replace your tyres then you should familiarise yourself with the tyre width, aspect ratio, rim diameter, load index and speed rating.

This is represented in the following way:

tyre sidewall numbers

  • 205 is the width of the tyre
  • 40 is the aspect ratio
  • 17 is the rim diameter
  • 84 is the load index
  • W is the speed rating

How do I know if my tyre size is right?

Tyre sizes vary so it is important to check the size your car requires before purchasing a tyre, you can either check the existing tyres on your vehicle and buy like for like or check your handbook to ensure you are purchasing the correct size.

The width of the tyre is measured in millimetres. In the case above, the tyre is 205mm wide across the tread.

The aspect ratio often referred to as the profile, is displayed as 40 above. It is measured in percentages; this tyre’s sidewall height is 40% of its width.

The rim diameter is the size of the rim measured in inches.

The load index is a numerical code that indicates the maximum weight capacity of the tyre. If you are looking for more information on the maximum weights each load index represents, simply click on the button below:

Related: Load index information

A tyres speed rating is represented as a letter, in this case above it is a ‘W’. The ratings represent the maximum speed a tyre can safely travel at. For example, a tyre rated W is for cars with a maximum speed of 168mph.

Between the aspect ratio and the diameter, you will usually find the letter R. This stands for ‘radial’ and relates to the construction of the tyre. Most new car tyres are radial tyres.

Choosing a tyre size for your car's specifications

It is crucial that you select a tyre in the correct size for your vehicle. Fitting a tyre that is incorrectly sized for your vehicle could result in a change to the ride height and the readings of the speedometer.

Speed ratings should be checked to ensure they comply with the vehicle manufacturers approved rating for the vehicle. Each letter denotes a specified maximum speed the tyre can travel at.

Choosing a tyre size for your car's load capacity

The load index of a tyre is the maximum capacity the tyre can carry when inflated correctly. Overloading a vehicle can be dangerous, the pressure it adds to the tyres can impact the car’s handling, increase fuel consumption and occasionally lead to accidents.

If in any doubt, our friendly staff are on hand to assist you. Simply call the Freephone number at the top of this page to be directed to your local branch. Or enter your registration in our tyre search facility to find the correct size for your car. We do however recommend that you check the size against the tyres currently fitted to your vehicle.

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