How To Read A Tyre: Understanding Tyre Size, Load Index, And Speed Rating

Every tyre sold by Tyre Shopper has markings on the tyre sidewall to indicate key information. This system of markings indicates the tyre size, tyre speed rating, model, and load index. Both low-profile and regular tyres use this marking system as standard, so it’s important that you know how to read and understand tyre markings.

According to TyreSafe, there has been 5,375 total casualties from tyre-related car accidents in the last five years and tyres and wheels caused 10.4% of failed car MOTs in the UK in 2017. Make sure you know how to buy the right tyres and check their key information with this guide.

How to read tyre size

With a little reading, it's easy to understand what the series of letters and numbers mean on your tyre's sidewall. Taking the image above as an example, we can work out vital information about the tyre that you will require if you ever need to replace it.

If the tyre reads 255/40 R20 101V:

What is the tyre load index rating?

Some vehicles are able to carry greater loads and therefore need a higher inflated tyre pressure reading. Tyres designed for higher loads bear the markings XL (Extra Load) or RF (Reinforced).

The tyre load index is the maximum capacity that each individual tyre can carry. It is usually represented in kilograms.

Is a higher tyre load rating better?

The higher the tyre rating is on the load index; the more mass the tyre can carry. A higher load index rating means that the tyre is stronger, and therefore can carry more weight. This also allows the tyre to take a higher amount of air pressure (PSI), allowing for heavier loads.

Whilst the load rating doesn’t make a tyre perform better in terms of grip and fuel efficiency, it will allow for heavier loads to be carried.

Does tyre load index matter?

The tyre load rating is crucial when it comes to tyres. If your load is too heavy for the tyre, then the tyre will struggle to perform. This could lead to an increase in wear, a higher risk of punctures, and less grip. Your car insurance policy may also be void in the event of a claim if you are found to have tyres with a lower load index rating the recommended for your car.

You should never go below the recommended tyre load rating for your vehicle, although it is safe to go for a higher rating. If there are no tyres available to match your current load rating, make sure you choose a tyre with a higher load rating.

How to calculate tyre load capacity?

A tyre load rating shows details the amount of mass the tyre can carry. In order to find your tyre's load rating, you’ll need to look at your tyre’s sidewall. Here, you’ll see characters in a similar format to 205/45 R 16 78 W. The second to last set of characters details the load rating, which in this example is 78.

To find out which number correlates to what weight, take a look at the tyre load rating index table above.

Does tyre pressure change with load?

When driving with an increased weight within the vehicle, it is crucial to ensure your tyres are inflated to the correct pressure. A heavy load will not change the tyre pressure significantly, as long as the tyres are the correct load rating for the vehicle.

To find the correct tyre pressure for your vehicle, you can look in the car’s handbook, inside the fuel cover, or on the inside of the passenger door. If tyres are underinflated, they will provide less grip and wear faster, which could compromise the safety of the car, especially under an increased load.

What does tyre speed rating mean?

Speed rating refers to the maximum speed a tyre can sustain under its recommended load capacity. These ratings are based on extensive testing carried out by the vehicle manufacturer, and the speed rating should not be exceeded.

Generally, the higher the tyre speed rating, the better quality the tyre. Being aware of your tyres' speed rating allows you to determine the maximum speed your tyre is capable of safely maintaining and is important when deciding upon new tyres. If you choose a tyre with a lower speed rating than is recommended, your insurance policy may be invalidated in case of an accident.

Tyre sidewall heights

Low profile tyres have a slightly different marking system to regular tyres. After the tyre section width figure, you'll find a number which indicates the sidewall height.

For example, in 255/40 R20 101V, the 40 means that the sidewall height is 40% of the tyre section width.

With a standard tyre, the sidewall height is usually 82% of the nominal section width.

Can I use a different tyre size?

Drivers should use the tyre size that is specified for the wheels on their car, with the profile and width that is specified in the car’s manual. However, a change in tyre size is possible when bearing in mind the wheel offset/inset value. This will help to avoid over-sized wheels and tyres becoming a hindrance to the functioning of the wheel arch or vehicle suspension.

When changing a tyre size, you should ensure that the tyre and wheel assembly diameter of the same value so that the speedometer and gearing remain unchanged. For each 10mm boost to tyre width, a reduction of 5% in the height of the sidewall is necessary, if the wheel size is unchanged. Of course, the tyres and wheels that you have selected must fit under the wheel arch without making contact with it or disturbing the vehicle’s suspension.

Our expert guidance for tyre sizes

Tyre Shopper can provide expert advice to buyers looking for tyres online, whether to change their tyre size or wheel size, so contact us today. We have 230 fitting centres across the country.

Please note that such changes are considered a modification by some UK insurance companies.

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Tyre Sizes Explained provides you with further information to help you understand which tyre size you will need for your vehicle.