Speed From RPM
Calculator
Results
- Speed (km/h)
- 104.499292
- Speed (mph)
- 64.932849
- Wheel speed (rpm)
- 877.192982
- Tyre circumference (m)
- 1.985486
Results
| Speed (km/h) | 104.499292 |
| Speed (mph) | 64.932849 |
| Wheel speed (rpm) | 877.192982 |
| Tyre circumference (m) | 1.985486 |
formula-map diagram
- Speed (km/h)
- 104.499292
- Speed (mph)
- 64.932849
- Wheel speed (rpm)
- 877.192982
- Tyre circumference (m)
- 1.985486
Formula breakdown
Formula
v = rpm ÷ (gear × final drive) × π × d × 60 ÷ 1000= 104.4992924773
Note
Simplified model: results use idealised textbook relationships and ignore real-world factors such as driving style, load, temperature, aerodynamic and rolling losses, drivetrain efficiency, tyre and brake condition and road surface. CO2 figures are tailpipe only, based on the standard 2.31 kg CO2 per litre of petrol (2.68 kg per litre of diesel), and exclude fuel production and distribution. Use them as estimates, not as legal, safety or load-rating figures.
More in Automotive and transport
See all →Frequently asked questions
What does this calculator determine and what inputs does it need?+
It calculates a vehicle's road speed at a given engine RPM, based on the gear ratio, final drive (differential) ratio, and tire diameter or circumference. These together determine how many times the wheels rotate for each engine revolution, and therefore how fast the vehicle moves.
How is speed calculated from RPM, gear ratio, and tire size?+
Wheel RPM = engine RPM ÷ (gear ratio × final drive ratio). Then speed is derived from wheel RPM and tire circumference: speed = wheel RPM × tire circumference × a time-unit conversion factor. Larger tires or lower (numerically smaller) gear ratios both increase speed for a given engine RPM.
Why does changing tire size affect this calculation even if gearing stays the same?+
A larger-diameter tire covers more ground per rotation, so for the same wheel RPM the vehicle moves faster; a smaller tire covers less ground per rotation and slows the effective speed. This is why changing tire size can meaningfully affect a speedometer's accuracy and the vehicle's effective final gearing.
What's a common misconception about gear ratio numbers?+
A 'higher' numerical gear ratio (like 4.10 versus 3.55) is actually a numerically lower gear in driving terms — it produces more wheel torque but lower speed per RPM, while a 'lower' numerical ratio (like 3.08) produces less torque but higher speed at the same RPM. Bigger numbers mean shorter (more torque-heavy) gearing, not taller gearing.
Why might my calculated speed differ from what my speedometer shows?+
Speedometers are calibrated for the factory tire size and gearing; if tires have been changed to a different diameter than stock, or the final drive ratio has been changed, the actual speed will differ from the speedometer reading, which is still calibrated for the original setup.