You use rollout if you run foam tires.

(Tire diameter x 3.14) / (Spur/pinion x internal gear ratio)

It's a calculation that tells you how far the car moves with 1 revolution of the motor. You can also use a tape measure and just turn the motor 6 knotches(1 revolution) and you'll get the same rollout(just take up the slack in the drivetrain when you measure).