Tyre dimensions explained

Understand your tyres better with this guide to dimensions, what they mean and how they affect the way your car drives

What are tyre dimensions?

You may not have even noticed but the sidewalls of your tyres are packed with information, much of which plays a big part in how your car behaves on the road. Understanding this information won’t change your life, but it is useful to know what the numbers mean especially when it comes to purchasing new tyres in future. Amongst all the information displayed on the sidewalls of a tyre the dimensions are usually the most prominent characters, and will be shown in a format like this - 195/65 R15. As far as the tyre’s dimensions are concerned, this is the only information you need.

How do I read my tyre size?

The first number in the sequence is the tyre’s width measured in millimetres, so in this example we can tell that the tyre is 195mm wide - a common width for superminis and smaller family cars. The next number shown is the tyre’s height, but this is expressed as a ratio rather than a measurement. So in our example the height is 65% of the tyre’s width - 65% of 195mm is 126.75mm. The R in the sequence indicates that this is a radial tyre, like the vast majority of modern tyres, and the final number is the inside diameter of the tyre measured in inches - this reflects the size the wheels fitted to your car, also measure in inches.

What does the tyre size mean?

The size of the tyres on your car contributes significantly to the way it behaves on the road, which is why it is so important to choose the correct size for your car when replacing them - or to make careful consideration should you wish to change the size of the wheels on your car.

The width of a tyre dictates how much grip is available for cornering, braking and acceleration. The wider a tyre is, the bigger the contact patch - the bit of the tyre in contact with the road - and the bigger the contact patch, the more grip is available. Although a lot of other factors are involved, a wider tyre will in general have a higher rolling resistance, which is a trade-off for the increased grip. This is why cars with modest performance and high economy have narrower tyres for efficiency, whereas high-performance cars have wider tyres and prioritise grip over efficiency.

090811proty

The aspect ratio of a tyre also has an effect on the way your car behaves on the road. The higher the aspect ratio the taller the tyre is in relation to its width, or how tall the sidewall is. A tall sidewall contributes to ride comfort by helping to absorb bumps in the road, but the trade-off here is that a tall sidewall deflects more during cornering, giving less accurate response through the steering wheel. A high-performance car will usually wear a low aspect-ratio tyre for this reason, while economy or comfort-biased cars will have higher aspect ratios.

As a comparison, prices for the Michelin Energy Saver+ were checked in a variety of sizes:

Tyre size

Michelin Energy Saver +

165/70 R14 B

£60.99

175/65 R14 T

£58.99

175/65 R15 H

£69.00

195/65 R15 H

£64.50

195/55 R16 T

£109.00

195/55 R16 V

£111.56

 

A brief snapshot of the available tyre sizes and prices shows that a number of factors contribute to the difference in price between one size and another. Although broadly speaking a wider and lower profile tyre will cost more than a narrower one, the popularity of a particular size can also play a big part in the cost.

Can I change the size of my tyre?

The short answer to this is yes, but there are several qualifiers. You may wish to fit a different tyre size to your car because you want to fit different alloy wheels, or you may wish to try and improve the performance or economy of your car. However, you can severely compromise the performance of your vehicle if you fit a tyre that does not suit it. The speedometer, gear ratios and some electronic safety systems are designed and calibrated to work with original equipment tyre sizes, so changing them can affect their behaviour.

If you do wish to change the wheels and tyres on your vehicle a shortcut to avoiding problems of this nature is to choose a wheel and tyre combination that is fitted to other versions of your car. Many popular models will be offered with a variety of wheel sizes depending on the specification, so you can upgrade safe in the knowledge that your car will continue to function properly.

Ask HJ

Will fitting slightly smaller winter tyres on my car affect it?

I have some wheels from my 2006 Nissan X-Trail - 215/60 R17 with winter tyres that I swap on and off. My new 2014 X-Trail has 225/55 R18. There's a small difference in the size but does it matter (especially if it's the rear wheels)
If the X-Trail is 4WD then all of the wheels and tyres need to be the same size and type and within 2mm of the same amount of wear otherwise this will throw the AWD system into confusion and will damage it. Also, never ever, put cold weather or all weather tyres only on the driven wheels of a car. They must be fitted to all four wheels otherwise, in extreme conditions, the car could come off the road.
Answered by Honest John
More Questions

* Sometimes we show a link with a * next to it. It means that it is an affiliate link and as a result helps HonestJohn.co.uk stay free to use. It's tracked to us and if you go through it, it can sometimes result in a payment or benefit to the site.

 

Search Good Garages