RC Gear Ratio & Top Speed Calculator
Figure out your car's final drive ratio and estimated top speed from your pinion, spur, motor Kv, battery voltage, and tire size — no math required.
Theoretical speed assumes 100% drivetrain efficiency, which no car actually achieves. The estimated figure applies a rough 15% efficiency loss (18% for 4WD, to account for the extra diffs and driveshafts) to get closer to what you'd see in practice. If you enter a weight, we nudge that estimate further based on rolling resistance — heavier cars lose more of their power just fighting friction, lighter cars lose less. This is still a rough approximation, not a full physics simulation: actual results also depend on your motor's torque curve, battery sag, tire traction, and surface.
How this works
Your final drive ratio (FDR) is simply your spur gear's tooth count divided by your pinion's — a 87-tooth spur and 21-tooth pinion gives an FDR of about 4.14:1. A higher FDR means more torque and acceleration but a lower top speed; a lower FDR (fewer spur teeth relative to pinion) means the opposite.
From there, your motor's Kv rating (RPM per volt) times your battery voltage gives motor RPM. Divide that by the FDR to get wheel RPM, and multiply by your tire's circumference to estimate how fast the car is actually moving.
Real cars never hit their theoretical top speed — motor and drivetrain losses, tire traction, weight, and battery voltage sag under load all eat into it. This calculator shows both numbers so you have a ceiling to compare against and a more realistic estimate.