How do I know if my car tyres are different sizes?

But for cars that haven’t been specifically engineered in this way, having different tyre dimensions can be dangerous.

Sometimes the difference is a matter of the profile: the back tyres are sometimes wider on performance cars. (Profile, and how to read it, is explained further down in our quick guide to reading tyre sizes.) But if you’re not driving a car designed for differential tyre sizing, don’t be tempted to replace a failed tyre with one that’s “more or less” the same. You could be putting yourself and your passengers in jeopardy.

Do tyre sizes matter that much?

The short answer is: yes. The safest configuration is to have four matching tyres, correctly aligned to the size of the rim. Cars that are specifically designed to have differently sized tyres should still have tyres of the same tread pattern and age. Major disparities between tyres can be dangerous because the car’s steering response can be altered. Having a single tyre that is wider than the others can seriously affect your control of the car in adverse weather conditions.
 
Luckily, finding out the tyre sizes is easy - just look at our easy-to-follow guide below. Once you have figured out the size, check your car’s manual to make sure that the tyres that are currently fitted are the correct ones for the size of the wheel. Or you can simply measure them and compare the rim size with what’s marked on the tyre.
 
If you’re worried about your tyres, or think the wrong ones have been fitted, talk to your local Protyre garage where you’ll get sound, unbiased advice from people who are experts at helping motorists drive safely and economically.

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How to read tyre sizes

For those who are new to this area, the side of the tyre is called the sidewall. This is where you’ll find the markings that give you a full rundown of the tyre’s size.

Width

The first three numbers on the tyre (for example, 225), tell you the width. This is measured in millimetres from one sidewall to the other, across the tread.

Profile or aspect ratio

The tyre’s profile is given in the two numbers that follow the width. It’s a percentage calculated using the width and height of the tyre’s sidewall. So a profile of 50% means that the height of the tyre’s sidewall is 50% of the tyre’s width. The commonest profile is ratio of 55 which means that the profile height of the tyre is 55% of its width.

Radial

These numbers are often followed by the letter “R”, short for “Radial”. That means the materials used to make the tyre are layered in such a way as to strengthen it. Nearly all tyres are radial today.

Size of the wheel rim

This is the diameter of the wheel that this tyre will fit. 16, for example, indicates 16”. Due to the long history of the automobile industry we’ve ended up with a mix of metric and imperial when describing tyres!