The pinion gear is the small gear on an RC motor's output shaft, held by a set screw. It drives the spur gear, and its tooth count is the primary adjustment racers use to change final drive ratio.
Bigger pinion means lower FDR, more top speed and more heat. Smaller pinion means higher FDR, more punch and a cooler motor. Pinion and spur must share the same pitch — 32, 48 and 64 are the common pitches today, while MOD gears have larger teeth and turn up on large-scale or heavy-duty vehicles. Team Associated’s B6.4 gear chart runs 48-pitch pinions from 20T to 33T, which shows the range you tune inside. Reset the mesh every time you swap: Associated’s manual says you should be able to rock the spur back and forth in the pinion teeth without moving the pinion. Too tight strains the motor; too loose chews tooth ends.
A pinion swap is the cheapest and fastest tuning change on the car, and a sloppy mesh after one turns a five-dollar gear change into a stripped spur mid-main.