RPM to Speed Calculator
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Speed from RPM, gear ratio, and tire diameter — imperial or metric.
Drivetrain
Road speed
67.9 mph
109.2 km/h
- Alternate units
- 109.2 km/h
Related calculators
RPM and road speed
Road speed depends on engine RPM, total drivetrain gear ratio (transmission × differential), and tire rolling diameter. Higher RPM or lower gearing increases speed for a given tire; larger tires raise speed at the same RPM.
This uses: speed (mph) ≈ (RPM × π × tire diameter in inches) ÷ (gear ratio × 1056). Enter total ratio — e.g., 3.73 diff only, or multiply trans gear × diff for in-gear cruising RPM checks.
Example: 3000 RPM, 3.42:1 ratio, 26 in tire → about 68 mph. Shift points and fuel economy planning often use this math — verify with a GPS for real-world calibration.
Common questions
What gear ratio should I enter?
Use the combined ratio for the gear you are in — differential ratio × transmission gear ratio. For final-drive-only checks, enter diff ratio and note it is an approximation.
Does tire pressure affect this?
Yes slightly — underinflated tires have smaller effective rolling diameter. Measure or use manufacturer spec diameter.