Wheel diameter and wheel radius

A car’s tyre must coincide with the diameter of its wheels. The specifications are provided on the sidewall of a tyre. For example, 205/45 R16 83V specifies the diameter of the tyre as 16 inches, which is (mentioned after ‘R’, that signifies the type of tyre construction. The wheel diameter is measured from lip to lip of a tyre, and coincides with the wheel rim size. 

The wheel rim size is specified to ensure the best option for ride comfort and drive dynamics. A small rim wheel fitted with a tall sidewall tyre provides a more cushioned ride but will mar handling, as the tall sidewall will cause the car to roll while cornering. 

A large diameter wheel fitted with a slim sidewalled tyre (aka low profile tyres) will make for a bumpy ride as the sidewall is unable to absorb the bumps. But such tyres enable fast cornering owing to minimised roll and better steering feedback. 

The wheel radius is half the wheel diameter, the distance from the centre of the wheel, the axle, to the outer edge.