The pinion is the small gear on the motor shaft that drives the spur gear. Its tooth count sets your gear ratio, which affects speed and acceleration.

Go smaller for more acceleration and cooler running, larger for more top speed. Match the pitch to your spur gear and check temperatures.

Match the pinion pitch to your spur gear: 48dp, Mod 0.6 or Mod 1 are common. Mixing pitches will not mesh correctly.