The three ratios that matter

An RC car drivetrain has three different numbers people casually call "gear ratio". Mixing them up is the fastest way to a cooked motor.

  • External gear ratio = Spur teeth ÷ Pinion teeth. Only counts the two gears you swap.
  • Internal ratio = the fixed transmission ratio inside the car (e.g. 2.72 on many 1/10 buggies). You cannot change it without changing the gearbox.
  • Final drive ratio (FDR) = external × internal. This is the number that actually decides wheel RPM per motor RPM.

What the FDR changes

A lower FDR spins the wheels faster per motor revolution — higher theoretical top speed, higher current draw and heat. A higher FDR accelerates harder and runs cooler but gives up top speed.

Worked example

87 T spur, 22 T pinion, 2.72 internal: external = 87/22 ≈ 3.95, FDR = 3.95 × 2.72 ≈ 10.76. The wheels turn once for every 10.76 motor revolutions.

Where it is used

Feed your FDR into the Gear Ratio Calculator and the Car Speed Calculator; helicopter drivetrains are staged differently — use the helicopter tool instead.