RC Car Gear Ratio Calculator

Gearing an RC car is a trade-off between top speed and torque, and this calculator turns your drivetrain numbers into both a final drive ratio and a rough top-speed estimate. It first divides your spur gear's tooth count by the pinion's to get the transmission ratio, then multiplies that by the differential's internal gear ratio to arrive at the final drive ratio — the total reduction between the motor shaft and the wheels. A higher final drive ratio means more torque multiplication (better acceleration and pulling power) at the cost of top speed, and vice versa for a lower ratio, which is why the same number is reported again as a "torque multiplier" to make that trade-off explicit.

Motor speed is estimated the simplest way possible: Kv rating (RPM per volt) times battery voltage, giving the motor's theoretical unloaded max RPM with no accounting for current draw, load, or heat — real-world RPM under load will be lower. That figure is divided by the final drive ratio to get wheel RPM, then converted to a theoretical top speed in mph using a fixed assumed tire circumference of 12.57 inches (a 4-inch diameter tire), so if your actual tires are a different size, the speed estimate will be off proportionally. Because it ignores motor efficiency, rolling resistance, aerodynamic drag, and battery voltage sag under load, treat the top-speed number as a ceiling rather than a real-world prediction — actual on-track speeds typically come in meaningfully lower.