Tyre Size Calculator


At Blackcircles.com we often receive queries regarding using a different tyre size on a car. Converting to a different size of tyre is possible, but it must be done within strict criteria. If you get it wrong, the consequences can be dire.

When a person is considering changing the wheel and tyre size of their car, it is important to remember that you will need the diameter to remain as close as possible to the original size.

Diameter Is Key

Why is the diameter so important? It is because speedometers, traction control, torque and the gear settings of a car are based on how far the tyre travels in a full revolution. This is linked to the outside diameter.

If the size is too different this affects the time/length it takes for a full tyre revolution (the tyre rolling circumference), which will cause your speedometer to display incorrect readings and your gearing will be adversely affected.

So, when changing the size of wheel and tyre fitted to your car, it needs to be the same overall diameter as what the car was designed to work with.

Unfortunately it isn’t immediately easy to work out the outside diameter when changing tyre sizes as tyre sizes are measured using the inside diameter.

On a tyre, the outside diameter is determined by the sidewall and is more commonly referred to as the aspect height.

Keeping the Same Diameter

To keep the same overall diameter, you need to make adjustments. If, for example, you were to move up a rim size from 16” to 17”, you will need to choose a tyre with an inch less in the standing height of the tyre.

Understanding Tyre Size Maths

This is where it gets a bit more complicated.

The aspect height of a tyre is shown in the tyre size as a percentage of the width - the aspect ratio.

SidewallLet’s take the tyre size 205/55/16 for example. 205 is the width of the tyre in millimetres – fairly straight forward. However, the 55 is the ratio of the tyre’s width to the tyre’s height.

So it means that the aspect ratio is 55% of the 205mm width. In this instance that is 112.75mm.

At this point you will need to convert the size from millimetres to inches – there are 25.4 mm in every inch.

To find out what the standing height is, you need to double the aspect height, (top and bottom of the sidewall) and then add on inside diameter – in this case 16”. So, in our example, the standing height will be 24.88 inches.

Now, that you have the standing height of the old tyre, you can work out which new tyre size will also work on your car –as the standing height will need to be the same.

Maths Summary:

  1. Multiply a tyre’s width by the aspect ratio to get aspect height (205 x 0.55 = 112.75)
  2. Convert into inches (112.75 / 25.4 = 4.44)
  3. Double the aspect height (4.44 x 2 = 8.88)
  4. Add inside diameter of tyre (8.88 + 16 = 24.88)

When changing tyre size, the tyres should not have more than a 1% difference in diameter - less than 0.5% would be ideal.

Important Safety Notice

Please do not fit a different sized tyre without first consulting an expert.

They will be able to tell you if you are trying to fit a tyre with a tyre size that could lead to an accident or damage to your car.

If you are interested in finding out more, call our team of experts on 0845 620 2000 and they will be happy to discuss the matter.

Please note - Blackcircles.com have created this page for guidance only. We strongly recommend checking with an expert before purchasing changing tyre size.

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