🌀 RPM Calculator
Calculate your engine’s RPM at a given road speed using tire diameter, transmission gear ratio, and final drive (axle) ratio.
What determines engine RPM?
At a given road speed, engine RPM (revolutions per minute) depends on how many times the wheels must turn to cover that distance, and how many times the engine must turn for each wheel rotation — set by the transmission’s gear ratio and the axle’s final drive ratio.
How this calculator works
This calculator first finds the total drivetrain ratio (Gear Ratio × Final Drive Ratio), then applies the standard formula: Engine RPM = (Speed in MPH × Total Ratio × 336) ÷ Tire Diameter (inches). The constant 336 converts miles-per-hour and inches into revolutions per minute, accounting for wheel circumference (336 ≈ (5280 × 12) ÷ (60 × π)).
Tire diameter should be the tire’s actual rolling (outside) diameter, not just its labeled size, since actual diameter affects the result. This calculator assumes no wheel slip or drivetrain backlash.
Use this tool for general gearing reference and educational purposes; always verify critical drivetrain calculations against your vehicle’s actual specifications.