Gear Ratio RPM & MPH Calculator
Calculate engine RPM at speed from gear ratio, final drive and tire size.
Gearing ties engine RPM to road speed. This calculator finds the RPM your engine turns at a given speed, based on your final drive ratio, transmission gear, and tire diameter — essential for choosing gears and reading cruise RPM.
Gear Ratio RPM Formula
How to Use This Calculator
- Enter the speed in MPH.
- Enter the final drive ratio and transmission gear ratio (1.0 for direct top gear).
- Enter tire diameter in inches.
Worked Example
Frequently Asked Questions
Use RPM = (MPH × final drive × gear ratio × 336) ÷ tire diameter in inches. The 336 constant converts the units to revolutions per minute.
Larger tires travel further per revolution, so they lower engine RPM at a given speed. Smaller tires raise RPM.
Use the transmission's top-gear ratio. Many manuals have a 1.0 direct top gear; overdrive gears are below 1.0 (e.g. 0.8).
A lower numerical final drive (e.g. 3.08 vs 3.73) reduces cruise RPM, improving fuel economy but softening acceleration.
Use the tire's overall diameter in inches, which you can get from the tire size markings or a tire-size calculator.