RPM Calculator
Find engine RPM from speed, or find vehicle speed from engine RPM.
Use this RPM Calculator to estimate engine RPM from vehicle speed or estimate vehicle speed from engine RPM using tire diameter and overall gear ratio.
RPM Calculation Method
The overall gear ratio represents the total ratio between the engine and drive wheels.
Overall gear ratio = transmission gear ratio × final drive ratio
Include any additional transfer-case ratio when applicable.
When tire diameter is in inches and speed is in mph, a common automotive formula is:
Engine RPM ≈ (vehicle speed × overall gear ratio × 336) ÷ tire diameter
To estimate vehicle speed:
Vehicle speed ≈ (engine RPM × tire diameter) ÷ (overall gear ratio × 336)
The calculator also uses:
Tire circumference = π × tire diameter
Engine RPM = wheel RPM × overall gear ratio
Example
Suppose a vehicle travels at 60 mph with 28-inch tires and an overall gear ratio of 3.50.
Engine RPM ≈ (60 × 3.50 × 336) ÷ 28
Engine RPM ≈ 2,520 RPM
The theoretical engine speed is therefore approximately 2,520 RPM.
Important Notes
- Use the overall gear ratio for the transmission gear currently selected.
- Use the tire’s effective rolling diameter when available.
- The calculation assumes no clutch, torque-converter, belt, or wheel slip.
- Real RPM or road speed can differ because of tire deformation, tread wear, loaded radius, drivetrain behaviour, and speedometer error.
- A calculated vehicle speed is theoretical and does not represent a safe, legal, or mechanically achievable top speed.