The lower the teeth on the pinion......gives you more torque, the more teeth gives you more RPMs.....but you can also over gear the motor.......
Trying to find the right ratio between pinion and spur for the track is somthing you have to play with. And also check your gear mesh.....make it too tight puts a strain on the motor as well causing the motor to work twice as hard and causing the motor to overheat.