We use roll-out to calculate our gear ratios. Its a simple formula, and is essential for proper gearing when running foam tires. Generally speaking, rollout is how far the car will move forward with one revolution of the motor. To compute the rollout, you divide the circumference of the tire by the total gear reduction of the car.
Rollout = Tire Circumference divided by the Total Gear Reduction
The first step toward computing your rollout is to figure out the circumference of your tires. This is easily done with a caliper. I do all of my calculations using inches, but some also use the metric scale (Millimeters). The reason I do the measurement in inches is because I have a general idea of what the rollout of my car should be in inches.
Measure one of your tires. Take that number and multiply that by PI (3.1416). The number you now have is your tire circumference.
Tire Circumference = Tire Diameter * 3.1416
Total Gear Reduction
Next, you need to figure out your total gear reduction. Take your spur gear (larger gear) and divide it by your pinion gear size (smaller gear on the motor). That gives you the gear reduction of the spur and pinion combination. In addition to the spur and pinion, a touring car will have some additional internal gear reduction. The internal gear reduction can be found by dividing the number of teeth on the differential by the number of teeth on the layshaft (belt drive car) or center shaft (shaft drive car). That gives you internal gear reduction. Now take the spur-pinion reduction and multiply it by the internal drive ratio of the car you have. That gives you the total gear reduction.
With my stock touring car, I like to gear my rollout at 0.90-1.10 inches. The exact rollout depends on which motor I am planning to run and how many RPM that motor has. The Epic Roar Stock likes a rollout of .90-.93. The Trinity Monster Stock motors like a rollout of .93-.97. The Trinity P2k and P2k2 motors are slightly lower RPM motors, so I gear the rollout for those at 1.0-1.10. If I use a Trinity Green Machine, I set the rollout to about .95-1.0. Each motor is a little different, so the goal is to merely get within the target rollout range for the type of motor. Then with some trial and error, you can find the perfect rollout for each motor you own.
Lets say I am driving an XRay T1FK05 in the stock touring car class, and I am running a Trinity Monster Stock as the motor. I measure the tires and find the diameter is 2.3228 (59mm). I take 2.3228 * 3.14 to find that my tire circumference is 7.294. Next I take my 120 tooth spur gear and divide by my 27 tooth pinion gear to find the spur-pinion gear reduction is 4.444. Then I look up my car in the table above to find that my internal gear reduction is 1.70. Then I multiply 4.444 * 1.70 to find that my Total Gear Reduction is 7.555. Then I divide my tire circumference of 7.294 by my Total Gear Reduction of 7.555 to find that my rollout is .9654. This may be a good starting point for a Trinity Monster Stock motor in Stock Touring Car or Sportsmans Touring Car classes. However, it may run better with a 26 tooth pinion, which would yield a rollout of .9296. Both of these pinions with place the rollout within the target rollout range for the motor. So how do I know which pinion to use? If I have time, I will try a few practice laps with one pinion and then a few laps with the other. If I dont have time to practice with each pinion, I will just make a guess.
I hope this article helps you compute your rollout.