How the pinion and spur set your gearing
The pinion is the small gear on the motor shaft; the spur is the big gear it drives. Spur ÷ pinion is your primary ratio, and multiplying by the car's internal (transmission) ratio gives the final drive ratio, FDR: how many motor turns it takes to turn the wheels once. A 78T spur and a 24T pinion on a 2.6:1 transmission is 78 ÷ 24 × 2.6 = 8.45.
Bigger pinion, smaller spur: what changes
Going up a tooth on the pinion lowers the FDR: more top speed, less punch out of corners, and more heat in the motor and ESC. Going down does the opposite. Because the pinion is cheap and quick to swap, it is the gear you tune with; the spur moves every ratio at once and gets changed when the pinion range runs out.
Heat is the limit
The right pinion is the biggest one that keeps the motor and the ESC in their temperature windows at the end of a run. Over-gear and both run hot: the motor loses power and the ESC can go into thermal shutdown mid-run. Under-gear and they run cool, the car accelerates hard and tops out early. Start a tooth conservative, run, check the motor can and the ESC, then step up. Hotter motors (lower turns, higher KV) need smaller pinions than mild ones in the same car.
Tight track or open track
A tight, technical layout rewards a lower gear (smaller pinion) for drive off the corner; a big open track rewards a taller gear for straight-line speed. The recommender shifts its baseline about 3% either way for that, the same nudge racers make by hand.
Pitch and bore
Pinion and spur must share a pitch: 48P is the 1/10 race standard, 32P and Mod 1 are common on 1/8 and RTR trucks, 64P on pan cars and Mini-Z. The bore must match the motor shaft (3.175 mm on most 1/10 motors, 5 mm on 1/8 and many RTRs). A wrong-pitch pinion will strip a spur in one run.
Where the recommendation comes from
These are the same recommendations you get in Pit Calc Pro and RC AI Mechanic. For each class (scale, drive, body style, track style) we look at how racers' gearing tracks their motor across real race setups, then apply that to your spur and internal ratio, and show the range racers with a motor like yours actually ran. If we don't have enough data for your class and motor, it will not give a recommendation: classes raced on turn-rated motors have no KV data, and bashing classes have little race data, so Traxxas and ARRMA trucks get one only where their class has been raced. The pinion table on every car page still shows what each tooth does to the ratio, and the gear ratio calculator handles any spur, pinion and tire.