Originally Posted by CraigV

That number is the number of times that the motor has to make a complete revoluotion for every revolution the tires turn.

For example, the SC10 4x4 has an internal transmission ratio of 2.57. If you were to use a 62 tooth spur gear and a 12 tooth pinion gear, the final drive ratio would be:

FDR = (spur / pinion) x Transmission Ratio
FDR = (62/12) x 2.57
FDR = 13.278

That means that the motor has to revolve 13.278 times for the wheels to revolve once.

Let's say we have a 3800 kilovolt motor running at 7.4 volts. The motor would be turning at (3800x7.4) = 28,120 RPM or rotations per minute. So, in one minute the motor will turn 28,120 times.

With a FDR of 13.278, the wheels will turn (28,120 / 13.278) = 2,117.79 times.

If we decrease the FDR by raising the number of teeth on the pinion gear by two, we get an FDR of (62/14)x2.57 = 11.38. This would mean that during the same minute that the motor turns 28,120 times, the wheels would turn 2,471 times, which is 353.2 times more often than with the smaller pinion gear.

Since the wheels are turning more often (and hence faster) during this minute, the motor is using more power (drawing more amps with the same amount of voltage). This causes the motor to get hotter, and your battery to drain sooner.

Craig