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RC Gear Ratio Calculator

Work out the external gear ratio and final drive ratio of an RC car from spur teeth, pinion teeth and the internal transmission ratio.

Gearing

Result

Final drive ratio
10.76: 1
93.0 wheel RPM per 1,000 motor RPM
External ratio
3.95: 1
Internal ratio
2.72: 1

Wheel turns once per shown number of motor revolutions.

Formula

  1. 1.External Gear Ratio = Spur Teeth / Pinion Teeth
  2. 2.Final Drive Ratio = External Gear Ratio × Internal Ratio
  3. 3.Wheel revolutions = Motor revolutions / Final Drive Ratio

Worked example

  1. 1.Inputs: 87 T spur, 22 T pinion, internal ratio 2.72.
  2. 2.External ratio = 87 / 22 ≈ 3.95
  3. 3.Final drive ratio = 3.95 × 2.72 ≈ 10.76
  4. 4.The wheels turn once for every 10.76 motor revolutions.

Assumptions and limits

  • A lower final drive ratio spins the wheels faster per motor RPM — more speed, more heat and current draw.
  • A higher final drive ratio trades top speed for acceleration and cooler running.
  • This tool covers car drivetrains. Helicopter gear reduction is a different layout — use the helicopter tool instead.

Related tools

Guides and concepts

FAQ

What is the difference between external and final drive ratio?

External ratio is only spur ÷ pinion. Final drive ratio multiplies that by the fixed internal gearbox ratio — it is the number that actually decides wheel RPM.

Bigger pinion = more speed?

On paper yes — a larger pinion lowers the external ratio and the final drive ratio. Watch motor temperature: gearing too tall can overheat the motor.

Assumptions

  • External ratio counts only the spur and pinion gears.
  • Internal ratio is the fixed gearbox ratio of the car.
  • Final drive ratio combines both stages.