Definition: On an electric RC, the pinion gear attaches to the shaft of the electric motor. The pinion gear meshes with the larger spur gear to provide power to the drivetrain.
Pinion gears come in different sizes -- number / spacing of teeth -- that matches up with certain sizes of spur gears. The gear setup (specific sets of spur gears and pinion gears) affects the speed and power of the RC. Some gear setups provide more top end speed but may be slow off the line while others get up to speed faster but may not be the fastest over the long haul.
Common Misspellings: pinon gear