Go-Kart Gear Ratio Calculator

A go-kart's gearing is set by two sprockets — a small one on the engine (the driver) and a larger one on the rear axle — and the ratio between their tooth counts (axle teeth divided by driver teeth) determines the tradeoff between acceleration and top speed. This calculator converts that ratio into real-world speed using the tire as the final link in the chain: at a given engine RPM, the wheel spins at engine RPM divided by the gear ratio, and each wheel revolution advances the kart by one tire circumference, so top speed falls out of wheel RPM times circumference, converted from inches per minute into miles per hour (dividing by 63,360 inches per mile and multiplying by 60 minutes per hour). Running that relationship in reverse gives the RPM needed to hit any target speed with your current gearing — useful for checking whether a target speed sits inside your engine's usable power band.

To recommend hardware, the calculator doesn't solve the ratio algebraically into a single answer; it brute-force searches every combination of driver teeth from 8 to 20 and axle teeth from 50 to 90, and returns whichever pairing lands closest to the exact ratio your target speed and max RPM imply, since real sprockets only come in whole tooth counts. The biggest source of error is tire circumference — it changes with tire wear, pressure, and load, so measure it directly around the mounted, loaded tire rather than computing it from a nominal diameter spec. This models a single fixed gear ratio and doesn't account for chain-drive losses, converter/clutch engagement RPM, or engine torque curve shape.