The ratio between the tire circumference and the gear ratio. For example:

30/120 (pinion/spur) on a pan car equals a 4.00 gear ratio. A 2.25" tire multiplied by 3.14 equals a 7.065" circumference.

7.065 divided by the 4.00 equals a 1.766 "roll out" which is how far in inches the car moves per motor revelation.

Spur ? Pinion = Gear Ratio
Tire Height x 3.14 = Tire Circumference
Tire Circumference ? Gear Ratio = Roll Out

120 ? 30 = 4
2.25 x 3.14 = 7.065
7.065 ? 4 = 1.766