Understanding Tyre Sizes

By: Stewart Creaser

If you’re looking to change your tyres then it’s important you buy the right tyre size for your vehicle. In addition as different tyres are designed to carry different maximum specified loads when operating at maximum given speeds, it’s also essential that you get a tyre that can cope with the load capacity your vehicle will generate.

You can find vital tyre information relating to size as well as load and speed capability on the sidewall of your tyre, in your vehicles handbook or a sticker in the door jam or on the inside of the fuel cap door. However the information is displayed as a tyre code, a series of different numbers and letters, and unless you know what you’re looking for it can be difficult to decipher. Our handy guide aims to help you decode this so that you can be better informed about tyre measurements next time your tyres need changing.

What do the tyre markings mean?

Tyre Wide Rating:

The tyre width rating is given in millimetres and is measured from the maximum width of the tyres midpoint of its sidewall to the same point on the opposite sidewall.

Wider tyres can offer better grip but can create excessive amounts of road noise.

Aspect Ratio:

Aspect ratio is also known as the tyres profile and relates to the depth of the sidewall. The height of the tyre's sidewall is expressed as a percentage of the tyres section width. It can be worked out by dividing the height of the sidewall by the width and multiplying it by 100.

Lower profile tyres can offer better handling and greater handling, however, this can often be at the cost of a slightly harder ride quality. Low profile tyres are often fitted on performance vehicles.

Radial Construction:

Most tyres are of a radial construction which is marked on tyres by a letter ‘R’. In radial construction tyres, the cord plies are arranged at 90 degrees to the direction of travel which allows the sidewall and the tread to function as two independent features. This makes them stronger and more flexible which makes them better at absorbing shocks, impacts and bumps.

Tyre Rim Diameter:

The tyre rim diameter is taken from the wheel flange where the tyre is seated to the same point on the opposite side. The tyre rim diameter measurement is given in inches.

Tyre Speed Rating:

Your tyre is given a maximum tyre speed rating for when it is operating at maximum load carrying capacity. It is represented on the tyre by a letter and measurements range from A1 to Z and cover maximum speeds of 5km/h to 300km/h.

 NB: Choosing a tyre with a lower speed rating than appropriate for your car can invalidate your insurance. 

Tyre Size Example

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Finding the Right Tyre for your Car

When buying new tyres it is important that you stick to the right tyre size. Speedometers, traction control, torque and gear settings are all based on how far the tyre travels in a full revolution which is directly linked to its outside diameter. If you fit the wrong sized tyre you may alter the time it takes for a full tyre revolution, and as a result, could cause the speedometer to display incorrect readings as well as adversely affecting your cars gearing.

At Stoneacre we have a state of the art search function which allows you to simply input your registration and it will suggest a variety of different tyres that will be the right size for your vehicle. This gives you complete peace of mind that you’re getting the right size tyre with the correct tyre rating for your vehicle.

However, if you know what tyres you’re looking for simply input the tyre measurments and our search function will offer up all available tyres that meet your requirements.

Please note we record all our calls to ensure that we give you the service you deserve.