How to use this calculator
- Enter road speed. Set the target cruise or test speed.
- Enter tire diameter. Use nominal tire diameter or measured rolling diameter.
- Enter ratios. Enter transmission gear ratio, final drive ratio and transfer ratio.
- Read engine RPM. Review engine RPM and speed per 1000 rpm.
How it works
A tire moves the vehicle one circumference per wheel revolution. The calculator finds wheel speed from wheel rpm = road speed x 60 / (pi x tire diameter), then multiplies by the drivetrain ratio: engine rpm = wheel rpm x gear x final drive x transfer ratio.
Use the tire size calculator if you are comparing tire changes first, or the gear ratio calculator when the question is a general gear pair rather than a vehicle cruise RPM.
Worked example
Verified against the live calculator
At 65 mph, a 26 in tire, 0.75 overdrive and 3.73 final drive give an overall ratio of 2.80. Wheel speed is about 840 rpm, so engine speed is roughly 2,350 rpm.
Frequently asked questions
How do you calculate engine RPM from speed?
Wheel RPM is road speed divided by tire circumference. Engine RPM is wheel RPM multiplied by transmission ratio, final drive ratio and transfer-case ratio.
Do overdrive gears use ratios below 1?
Yes. An overdrive transmission gear such as 0.75 means the engine turns 0.75 times for each transmission output revolution before the final drive is applied.
Does this include torque converter slip?
No. It assumes a locked converter or direct mechanical drive. Add slip separately for an unlocked automatic transmission.
Should I use tire diameter or revs per mile?
Measured rolling diameter or manufacturer revs-per-mile data is better for precise speedometer or cruise RPM work. Nominal tire diameter is a good first screen.
Method & assumptions
- Assumes direct mechanical coupling with no clutch slip, torque-converter slip or tire growth.
- Uses tire diameter as a circumference proxy. Manufacturer revs-per-mile data is better for calibration.
- Does not include drivetrain efficiency, aerodynamic load or shift strategy.