Gear-Ratio MPH from RPM
Road speed (or RPM), wheel RPM, and tire revolutions per mile from engine RPM, transmission and axle ratios.
Example
You enter
- Solve for mph
- Engine RPM 2500
- Transmission gear ratio 1
- Axle ratio 3.55
- Tire diameter (in) 28.5
You get
- Road speed (MPH) 59.71
Checked against published worked examples from: Drivetrain kinematics + SAE J267 tire convention.
Details, formula, and sources
ScopeRoad speed (or RPM), wheel RPM, and tire revolutions per mile from engine RPM, transmission and axle ratios, and tire diameter.
Formula or tableMPH = RPM * pi * dia(in) * 60 / (trans * axle * 63,360); inverse for RPM; revs/mile = 63,360 / (pi * dia).
Source and editionClassical drivetrain kinematics; SAE J267 metric tire-size convention for decoding a tire code to diameter, by name.
Free accessPure geometry, public; consistent with the tire-gearing tile.
What governsEstimate. AHJ and licensed professional govern.
API field namessolve_for, rpm, trans_ratio, axle_ratio, tire_dia_in, mph
- No slip geometric speed ignores tire and torque-converter slipdrivetrain kinematics