How to Find and Read your Tyre Size

If you are replacing the tyres on your vehicle or selecting new ones, you will need to ensure that the tyres are correct for the model of your vehicle. All tyres should have their specifications printed on the tyre wall, as per the diagram below:

Let’s take the Uniroyal Rainsport 3 225/40 R18 92W XL, FR, SSR, Run Flat for example (don’t let the numbers intimidate you). Let’s break it down by section:

225 Explained

This is the width of the tyre in millimetres, measure from one sidewall to the other. You may find that your tyres have a different width at the front and rear  – this is called a ‘staggered’ configuration. This typically occurs on vehicles with a mid or rear-engine configuration, operating on a rear-wheel drivetrain.

Thicker tyres offer better grip (through a larger road contact patch) than thinner tyres, however, thinner ones tend to offer better rolling resistance and economy. Rolling resistance is the energy your car requires in order to maintain momentum at a constant speed, by countering friction.

40 Explained

This is the profile, which relates to the height of the tyre’s sidewall – displayed as a percentage of the width of the tyre. This means that the tyre’s wall is 40% of its width. Some tyre manufacturers also call this the ‘aspect ratio.’ This can be calculated by dividing the tyre’s rim height by the width of the tyre.

Lower aspect ratio tyres can provide a superior handling experience; however, this generally sacrifices ride comfort. Tyres with a higher aspect ratio or profile will generally provide a more comfortable ride.

R18 Explained

This number relates to the diameter of the tyre’s inner rim and is displayed in inches. The bigger the rim, the larger and more expensive the tyres tend to be. Modern tyres are made using a radial (R) construction method.

92 Explained

Here we have the load rating of the tyre, which determines how much weight each tyre can carry. The numbers relate to specific weights so you will need to refer to this tyre load index table https://www.roundtriptyres.co.uk/tyre-guide/tyre-load-index/ to see which weight the various numbers relate to.

For example, 96 indicates that each tyre will support 710kg of weight. This is then multiplied by the four tyres (or more) that the vehicle has, giving you the maximum load the vehicle can support. This is one of the most important numbers in the sidewall sequence; if you have a crash or an accident and are found to be carrying more than the maximum load, your insurance and warranty could be considered invalid.

 

W Explained

The final letter indicates the maximum speed of the vehicle. You will need to consult the following guide in order to find out what speed the letter relates to: https://www.roundtriptyres.co.uk/tyre-guide/tyre-speed-rating/.

For example, V represents a maximum rating of 149 mph. This is extremely important because it tells you the maximum speed the tyre was designed to withstand. Going faster than this speed could cause the tire to fail.

Additional Letters and numbers 

You may find additional information on your tyre. For example, XL indicates that the tyre has been reinforced for heavier loads. FR is an abbreviation of Flange Rim, which means that the tire has additional protection against kerbside damage.

SSR stands for self-supporting run flat. Generally, when a tire is flat, it is impossible to drive on. Run-flat tyres or tyres marked with SSR allow the tyre to be driven on at 50 mph for a maximum of 50 miles (80 kilometers), so that the owner of the vehicle can safely drive to a garage to have the tyre replaced.

From a technical perspective, the run-flat technology prevents the sidewall of the rim from making contact with the ground, thus eliminating damage to the sidewall. This means you don’t have to carry a spare tyre.

What you prioritise when purchasing new tyres depends on your personal preference and how you want to use your vehicle – it is critical though that you understand the tyres that you’re buying in order to get the performance and safety you need from them. When searching for tyres, you can generally shop by the size of tyre for example 235 55r17 tyres, 245 45 r18 tyres or 195 55 r15 tyres

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