How to calculate rolling radius
Rolling radius describes the radius of a wheel measured from centre to tread. There are two types of rolling radius measurements used: static rolling radius and dynamic rolling radius. Static rolling radius is measured with the wheel at rest.
Dynamic rolling radius is measured with the wheel in motion, and is difficult for the average person to measure. There is generally a slight expansion of the radius witnessed when the wheel is in motion. For most purposes, static rolling radius is sufficient.
Stop the vehicle on a level, unobstructed surface.
Find the exact centre of the wheel. If this is not obvious, you can measure across the wheel until you find the longest measurement. This will be across the centre. Divide this measurement by two to find the centre point.
Measure from this centre point to the ground to determine static rolling radius.