How to use it
- Choose whether to find speed from RPM or RPM from speed.
- Enter the transmission gear ratio and the final drive ratio.
- Enter your tyre size; the circumference is worked out from it.
- Read the result, with the wheel speed and the circumference.
How it works
- Wheel RPM = engine RPM ÷ (gear ratio × final drive).
- Circumference = π × (rim × 25.4 + 2 × width × aspect ratio ÷ 100) mm.
- Speed in km/h = wheel RPM × circumference (m) × 60 ÷ 1000; mph = km/h ÷ 1.609344.
- RPM for a speed reverses the same steps.
Worked example
Top gear 0.80, final drive 3.42, 225/45R17 tyres (circumference 1.9927 m), at 3,000 rpm: the wheels turn 3,000 ÷ (0.8 × 3.42) = 1,096.5 times a minute, which is 131.1 km/h or 81.5 mph. For 100 km/h the engine needs about 2,288 rpm.
Tips
- Use the right gear. The ratio of the gear you are in sets the result; 1.00 is a direct drive.
- Tyre size matters. A different size moves every speed by its diameter ratio.
- A guide only. Slip and tyre squash cause a small difference on the road.