What gear ratio should I use?

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.

How does the pinion/spur combination work?

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.

What is the internal ratio?

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 pinion/spur ratio multiplied by the internal ratio.

How do I measure tyre circumference?

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.

ModelCalcs

Gear Ratio Calculator — RC Drive Train

Calculate gear ratios, final drive ratio and top speed for RC cars and trucks.

Pinion Teeth

Spur Teeth

Motor RPM (loaded)

Internal Ratio

From vehicle manual

Tyre Circumference (mm)

How We Calculate This

Calculates drive train gear ratios and theoretical top speed from pinion/spur gears, internal ratio and tyre size.

Formula

Gear ratio = Spur teeth / Pinion teeth
Final drive ratio = Gear ratio x Internal ratio
Wheel RPM = Motor RPM / Final drive ratio
Top speed (km/h) = Tyre circumference x Wheel RPM x 60 / 1,000,000

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify quantities before purchasing materials.