RPM ↔ Speed Calculator
Enter the transmission gear ratio, final drive and tire diameter. Convert RPM to speed or speed to RPM to estimate cruise revs, the speed you reach in a gear, or which gear you need at a target speed.
Calculate
Your values convert when you switch units.
Result
The formula
Overall ratio is the transmission gear times the final drive. The wheel turns once for every overall-ratio turns of the engine, and each wheel turn moves the car one tire circumference.
Worked example
- 3,000 RPM · gear 0.85 · final 3.42 · tire Ø 632 mm
- Overall = 0.85 × 3.42 = 2.907 · circumference ≈ 1.985 m
- Wheel RPM ≈ 3,000 ÷ 2.907 ≈ 1,032 · speed ≈ 122.9 km/h (76.4 mph)
Result: ≈ 122.9 km/h at 3,000 RPM · 41.0 km/h per 1,000 rpm
How RPM and speed relate
We treat the tire as a perfect rolling circle at the stated diameter. Slip, load squat, and indicated vs GPS speed are ignored.
Overall gearing
Transmission ratio × axle (final drive) ratio sets how many engine turns become one driveshaft/wheel turn. Overdrive gears are less than 1.0; first gear is typically 3–5+.
Tire diameter
Use overall diameter (not rim size alone). From a size like 225/45R17: Ø ≈ rim×25.4 + 2×(width×aspect/100) millimetres. Our tire size calculator can estimate diameter from stock and new sizes.
What we do not model
No torque converter slip, clutch slip, AWD transfer ratios, or speedometer calibration error from non-stock tires.
Interesting facts
Why overdrive matters
A 0.7 overdrive with a tall tire can drop highway RPM by hundreds of revs versus a 1:1 top gear — quieter and often more efficient.
Redline in gear
Multiply your engine redline through this formula in each gear to sketch theoretical top speed before aero and power limits intervene.
Diameter changes speedo
Larger tires raise true speed at the same indicated RPM/gear; smaller tires do the opposite. Pair this tool with the tire size calculator when swapping sizes.
Final drive swaps
Shorter (higher numeric) axle ratios raise RPM at a given speed — stronger acceleration, busier highway cruise.
Frequently asked questions
Divide RPM by overall gear ratio (gear × final drive), multiply by tire circumference, then convert revolutions per minute into distance per hour.
Owner’s manual, service data, or enthusiast forums for your gearbox code. Final drive is often stamped on the differential or listed as “axle ratio.”
Overall loaded/design diameter in mm or inches — not the rim diameter alone. Spec sheets and sidewall math both work.
Yes — choose Speed → RPM and enter road speed plus the same gearing and tire diameter.
Tire growth with speed, wear, pressure, and factory speedo offsets are common. This tool assumes a fixed diameter and no slip.
References
- Vehicle dynamics — speed and gearing Professional literature on driveline kinematics and rolling radius.
- Tire size geometry Passenger-car tyre designation and dimensional conventions (ISO 4000 series context).