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.