RPM Calculator
Calculate engine RPM at any road speed from gear ratio, final drive, and tire size — essential for gearing decisions.
RPM at Speed Calculator
Speed + Gears + Tyre → Engine RPM
How It Works
Wheel rotational speed is derived from road speed and tyre circumference. Engine RPM is wheel RPM multiplied by the transmission gear ratio and the final drive (axle) ratio.
How to Use This Calculator
- Enter the desired road speed.
- Select the speed unit (km/h or mph).
- Enter the overall tyre diameter in mm (look up in tyre size calculator).
- Enter the transmission ratio for the gear of interest.
- Enter the final drive (axle) ratio — found in the workshop manual.
Worked Example
Reference Table
| Final Drive Ratio | Application | Highway RPM @100 km/h (approx) |
|---|---|---|
| 2.73 | Overdrive highway gearing | ~1 700 RPM |
| 3.23 | Economy/long-leg | ~2 000 RPM |
| 3.55 | Balanced | ~2 200 RPM |
| 3.73 | Common road car | ~2 300 RPM |
| 4.10 | Short-gear performance | ~2 500 RPM |
| 4.56 | Truck/towing | ~2 800 RPM |
| 5.13 | Off-road/race | ~3 200 RPM |
Frequently Asked Questions
How do I find the tire overall diameter from a sidewall code?
Use the Tire Size Calculator — enter codes like 225/45R17. Overall diameter = rim diameter × 25.4 + (2 × width × aspect ÷ 100). For 225/45R17: 17 × 25.4 + 2 × 225 × 0.45 = 431.8 + 202.5 = 634.3 mm.
Why does changing the final drive ratio affect cruising RPM?
Final drive is a multiplier between the transmission output and the wheels. A taller (lower number) final drive reduces highway RPM, improving fuel economy but reducing low-end grunt. A shorter (higher number) ratio raises RPM, improving acceleration.
What are the risks of an over-revving engine from a tire change?
If you fit much larger diameter tyres without re-calibrating the speedometer, you will under-read speed. The engine will also run at lower RPM for a given true speed — generally benign, but it may affect performance in lower gears.
How do I use this for gear ratio selection when swapping gears?
Calculate the RPM at your desired cruise speed using the current setup, then use different gear ratios until you find one that hits that RPM at the same speed. This lets you optimize highway fuel economy or acceleration.
