Correct me if I am wrong but...
Roll out is how far the car moves for 1 rotation of the motor. Normally gear ratio alone would be sufficient for calculation purposes but with cars running foam tires the roll out changes as the tires wear down. So as the tire wears down the car goes a shorter and shorter distance for every revolution of the motor. Roll out is calculated as tire circumference divided by gear ratio. Typically we change the pinion gear to get close to our roll out goal. Changing the spur gear takes too much time for track side adjustment.