For example, 1/12 scale tire with 1.800" diameter, 30 pinion, 96 spur.
gear ratio = 96/30 = 3.200
(1.800 * 3.1416) / 3.200 = 1.767 rollout
For touring cars or other non-direct drive cars, factor in the 'gearbox' ratio for the specific car in with the spur/pinion gear ratio to get the final ratio.
For a car with a 'gearbox' ratio of 2.25 and the same spur/pinion as above...
(96/30) * 2.25 = 7.20 final gear ratio
Basically, rollout represents the distance traveled with one rotation of the motor
The above calculations are done in inches, but metric measurments can be substitued for the tire diameter, which will give your rollout in the same measurments.
If you do a little research on Hobbytalk, you will find rollout charts that Hank created that you can download.