The pinion gear on your RC car is the gear that is attached to your motor, and the spur gear is the gear that the pinion drives. The spur is sometimes integrated with a slipper clutch or with a differential in the case of a 4WD car.
The pinion and spur determine your car's gear ratio and are critical in tuning your car so it transfers power efficiently given your motor and your track.
Learn how to tune your RC car's performance using the gear ratio.
Also check out our Gear Recommender App to get pinion recommendations, and our Gear Calculator App to calculate gear ratios and rollout.