For a given spur size, the left hand column is pinion size, the top row is tire diameter, and the rest of the data is the rollout.
See this post for an editable rollout Excel spreadsheet
http://www.rctech.net/forum/showpost...postcount=2770
So, after entering your spur size, measure your tire diameter, go down that column until you find your desired rollout, and the go across to find the pinion you should use. The top chart is rollouts in mm, the bottom is rollouts in inches. Both tables have the tire diameter listed in mm and inches, so it doesn't matter what units someone else gives you the rollout in, you can easily find it.
Ask others at your local track what rollouts they are using, and you can gear your car the same regardless of your spur tooth count and tire size. All motors have an optimum rollout for a given track. Rollout, by definition, is the distance the car will travel for one revolution of the motor shaft. It takes into account the gear ratio (pinion / spur) and the tire diameter. As your tires wear, to maintain the same rollout, you should change the pinion.
Too low of a rollout, the motor will run cooler, but not be as fast on top end, but may accelerate faster. Too high of a rollout, the motor will run hot, top speed will be higher, but probably won't have as much punch in the infield.