RPM to Speed
Convert engine RPM into road speed using gear and final drive.
About RPM to Speed
Engine speed is divided by the gear and final-drive ratios to get wheel speed, then the rolling tire circumference translates that into road speed. The same RPM gives very different speeds in different gears, so this is the quick way to sanity-check gearing and cruise RPM choices.
Formula
RPM to Speed
mph = RPM x pi x tire diameter / (gear x final drive x 1056)
- final drive
- Fixed axle ratio that multiplies the gearbox ratio
Worked example
With the default values below, RPM to Speed returns Road Speed of 56.5. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Engine RPMminimum 0
- 2,500
- Tire diameterminimum 1 in
- 25.5 in
- Gear ratiominimum 0.01
- 0.9
- Final drive ratiominimum 0.01
- 3.73
Results
- Road Speed
- 56.5 mph
- Road Speed
- 90.9 km/h
- Wheel RPM
- 744.7 rpm
Step by step
- 1Values used
- 2RPM to Speed
- 3Road Speed= 56.5 mph
- 4Road Speed= 90.9 km/h
- 5Wheel RPM= 744.7 rpm
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is RPM to Speed calculated?
mph = RPM x pi x tire diameter / (gear x final drive x 1056) Engine speed is divided by the gear and final-drive ratios to get wheel speed, then the rolling tire circumference translates that into road speed.
Why does RPM to Speed matter?
The same RPM gives very different speeds in different gears, so this is the quick way to sanity-check gearing and cruise RPM choices.
What values do I need to enter?
This calculator takes 4 inputs: Engine RPM, Tire diameter, Gear ratio, Final drive ratio. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.