What this converter does
This converter turns wheel RPM into road speed in miles per hour and back. Speed depends on wheel size, so enter the wheel or tyre diameter — a larger wheel travels further per revolution. Type the RPM and read the speed as you type.
Useful for gearing, dyno work and tyre changes. The default 25-inch diameter suits a typical car tyre; enter your own for bikes, karts or machinery.
The units it covers
Rotational speed becomes road speed only once the wheel size is known — a bigger wheel covers more ground per turn, so diameter is required.
View all units & their values
| Unit | Symbol | Value | Mainly used |
|---|---|---|---|
| Rotational speed | rpm | N | Wheel revolutions per minute |
| Road speed | mph | v | Miles per hour |
| Wheel diameter | in | d | Tyre/wheel outside diameter |
The formula
Road speed is the wheel circumference times revolutions per minute, converted to miles per hour:
mph = π × d × rpm ÷ 1056Where:
- rpm = wheel revolutions per minute
- d = wheel diameter in inches
- 1056 = 63360 in/mile ÷ 60 min/hour
Worked example
Find the road speed at 500 RPM on a 25-inch wheel.
mph = π × d × rpm ÷ 1056π × 25 × 500 ÷ 1056 ≈ 37.2 mphSo 500 RPM on a 25-inch wheel is about 37 mph.
The units in this example
How many times the wheel turns per minute. More RPM means more speed for a given wheel size.
- 500 rpm, 25 in ≈ 37 mph
- 1000 rpm, 25 in ≈ 74 mph
- rpm = mph × 1056 ÷ (π d)
- diameter matters
Linear speed along the road. Found from wheel circumference times revolutions per minute.
- 37 mph ≈ 500 rpm (25 in)
- 60 mph ≈ 806 rpm (25 in)
- mph = π d × rpm ÷ 1056
- bigger wheel = fewer rpm