it just an equation to work out how far one turn off the motor move the car. mm per Rev it help find the ideal gear ratio to keep the car going fast without dumping at before 8 min and stop cooking the motors. beacause the cars are direct drive the final drive will the circumferance of the tyre.

diameter of tyre *Pi = circumferance (Pi = 3.141)

divided by

spur/pinion = gear ratio