The tyre size calculator will give approximate diameter and circumference of the tyre size entered.

To calculate the effect on the speedometer of different tyre sizes the only measurement required is the circumference. This will provide a relatively accurate difference between two given tyre sizes. Inaccuracies come from the flexible nature of the material the tyre is constructed from. As circumference is directly proportional to both radius and diameter, any of the three measurements can be used in this calculation.