RPM to Speed Calculator
Every Gear Change Trades RPM for Speed
Engine RPM alone says nothing about road speed until it's translated through two more numbers: the gear ratio currently engaged and the final drive (axle) ratio behind it. A tall final drive spins the wheels slower for a given engine speed, favoring fuel economy at highway RPM; a short one spins them faster, favoring acceleration. Add tire diameter, which sets how much distance one wheel revolution actually covers, and the full chain from crankshaft to pavement is complete.
The Formula
Speed = Wheel RPM × 60 × Wheel Circumference
Wheel circumference is derived from tire diameter (π × diameter), then converted to miles or kilometers so the result comes out directly in mph or km/h.
Where This Matters
- Gear ratio selection — when swapping a rear differential or transmission, this calculation shows exactly what RPM the engine will sit at during highway cruising with the new ratio.
- Tire size changes — fitting larger or smaller diameter tires changes effective gearing even if the mechanical ratios stay the same, which this formula captures through the tire diameter input.
- Setting a shift point — knowing wheel speed at a given RPM in each gear helps plan shift points for both fuel economy and performance.
- Speedometer verification — after a gearing or tire change, comparing calculated speed against indicated speed reveals how far a speedometer has drifted from accurate.
Speed at Various RPM (3.73 Final Drive, 26″ Tire, Direct Gear)
| Engine RPM | Estimated Speed |
|---|---|
| 1,500 | 31.1 mph |
| 2,000 | 41.5 mph |
| 2,500 | 51.8 mph |
| 3,000 | 62.2 mph |
| 3,500 | 72.6 mph |
| 4,000 | 82.9 mph |
Computed directly from the calculator's formula at a 1:1 (direct) gear ratio, 3.73 final drive ratio, and 26-inch tire diameter.
How to Use This Calculator
- Select Imperial (inches, mph) or Metric (mm, km/h) units.
- Enter engine RPM and tire diameter for the selected unit system.
- Enter the transmission gear ratio for the gear you want to check.
- Enter the final drive (axle) ratio.
- Select Calculate to see the resulting road speed.
Related Calculations
Look up the ratio itself with the Gear Ratio Calculator, or check engine speed directly with the RPM Calculator.