RC Gear Ratio: How to Choose Your Pinion and Spur

Gearing is one of the adjustments that changes an electric RC car’s behavior the most. With the same motor and the same battery, swapping a pinion that costs a few dollars can give you more top speed, more acceleration or, if you overdo it, an overheated motor. Here’s how it works and how to choose well.

Pinion, spur and internal ratio

• Pinion: the small gear mounted on the motor shaft.
• Spur gear: the large gear driven by the pinion.
• Internal ratio: the reduction added by the transmission or the differentials. Your car’s manual lists it.

The final drive ratio (FDR) sums up all three: FDR = (spur teeth ÷ pinion teeth) × internal ratio. A high FDR (“shorter” gearing) gives more acceleration and less top speed; a low FDR (“taller” gearing) does the opposite.

What changes when you go up or down on the pinion

Change	Top speed	Acceleration	Temperature
Larger pinion	Up	Down	Up
Smaller pinion	Down	Up	Down
Larger spur	Down	Up	Down
Smaller spur	Up	Down	Up

You can see the effect in numbers with the RC car speed calculator: enter your motor, battery and gears and try different combinations.

How to choose the right gearing

1. Start with the recommended gearing in the car’s or motor’s manual for your battery.
2. Change one or two teeth at a time. Small changes, big effects.
3. Watch the temperature. After a run of a few minutes, the motor shouldn’t be too hot to touch. If you can’t keep your finger on it for a few seconds, gear down or go back to the previous setup.
4. Think about the surface. On small tracks with tight corners, shorter gearing works better. On long straights or in open spaces, go taller.
5. More cells, shorter gearing. If you move from 2S to 3S, the motor spins faster, so you’ll usually need a smaller pinion to keep it from overheating.

Gear mesh: not too tight, not too loose

When you change the pinion, you need to set the distance between pinion and spur:

• Too tight: it drags on the drivetrain, heats the motor and drains the battery.
• Too loose: the teeth skip and the spur gets stripped.

The classic trick: put a strip of paper between the pinion and spur, slide the motor in until it pinches the paper, tighten the motor mount screws and roll the gears to pull the paper out. It should come out marked but not torn. Then check that there’s a little play when you rock the spur with your finger.

Pinion compatibility

The pinion must match the spur in module or pitch (the most common are Mod 1, Mod 0.8, 32 pitch and 48 pitch) and the motor in shaft diameter (3.17 mm and 5 mm are typical). If you’re not sure, check the manual or the spur’s part number.

Gearing and run time

Shorter gearing reduces current draw and stretches your run time. Estimate the difference with the RC battery run time calculator. And if you want to understand the other half of the equation, compare brushless vs brushed RC motors and review what an ESC is.