Gear Ratio Calculator — RC Drive Train

Lower gear ratios (smaller number) give more top speed but less acceleration and torque. Higher ratios give more acceleration and torque but lower top speed. For general bashing, a moderate ratio works best. For racing, tune the ratio to match the track layout.

The pinion gear sits on the motor shaft, the spur gear on the drivetrain. A 20T pinion with 80T spur gives 80/20 = 4:1 ratio. Increasing pinion size (or decreasing spur) lowers the ratio, increasing top speed. Decreasing pinion raises the ratio for more torque.

The internal ratio is the fixed gear reduction inside the transmission/differential. Typical values are 2.0-3.0. Check your vehicle manual for this specification. The final drive ratio is the spur/pinion gear ratio multiplied by the internal ratio.

Wrap a piece of string around the tyre at the contact point, mark where it meets, then measure the string length. Or measure the tyre outer diameter and multiply by Pi (3.14). Typical 1:10 RC car tyres have a circumference of 180-220mm.