The easiest way to determine what gear ratio you
should start with would be to find someone at the track that has a similar setup and ask him/her what they are running in their car...
If not, just experiment. Start out at a low gear ratio (weird how "low" gear ratio means a higher number, i.e. 12:1 is a "lower" gear ratio then a 9:1
)
If you find yourself constantly topping out in speed (meaning, your car won't go any faster), you have too much acceleration, AND your motor is relatively cool (meaning it's hot but doesn't burn your fingers when you touch it after one battery pack), then you should up a tooth on the pinion. Always step up/down pinions with one to two teeth. One tooth or two teeth in pinion makes a DRASTIC difference in the way the power/speed feels on your car and the temperature of your motor...