Tyre Calculator

This page allows you to calculate the overall diameter of a tyre from its specifications and allows you to see the effect of tyre changes to speedometer readings.

Section width refers to the width of the tyres, measured in mm. Aspect ratio is the ratio of the height of the tyres to the width - a ratio of 50 gives a tyre that is half as high as it is wide. Rim diameter is the diameter of your wheels, given in mm.

A tyre marking of 185/65R15 refers to a section width of 185mm, an aspect ratio of 65 and a rim diameter of 15".

Remember that not all tyre sizes are available.

Current Tyre Details

Select the specification of your current tyres.

Replacement Tyre Details

Select the specification of your replacement tyres.

Difference between Current and Replacement Tyres