Rollout is the simple name for calculating how many millimetres the car will travel per one motor revolution. It is mainly used for recalculating gear ratios for foam tyres because as they wear they get smaller.

The formula is:

Tyre diameter multiplied by pi (3.14159) divided by overall gear ratio

example: 63mm*3.14159/6.5= 30.44 rollout (30.44 millimetres per motor revolution.)