How this calculator works
Engine speed and road speed are linked by the tire circumference and the overall gear ratio. One wheel revolution moves the car one circumference, so road speed divided by circumference gives wheel revolutions per second, which becomes wheel rpm. Multiplying wheel rpm by the overall gear ratio gives engine rpm. The overall ratio includes the gearbox and the final drive, and a torque converter or clutch slip will make the real engine speed slightly higher than this rigid calculation.
Formula
wheel rpm = speed / (pi x tire diameter) x 60. engine rpm = wheel rpm x overall ratio. speed = wheel rpm / 60 x pi x tire diameter. All lengths must be in the same unit.
Worked example
A 700 mm tire has a circumference of 2.199 m. At 100 km/h (27.78 m/s) the wheels turn at 757.7 rpm. With an overall ratio of 3.5 the engine is at 2,652 rpm. At 3,000 rpm in the same gear, the car would be travelling at 113 km/h.
Advertisement
Frequently asked questions
What is the overall gear ratio?
It is the gearbox ratio multiplied by the final drive ratio. If the gearbox is in a 1.00:1 gear and the differential is 3.50:1, the overall ratio is 3.50:1. A lower gear has a higher number.
Why does the calculated RPM differ from the tachometer?
Tire rolling radius changes with load and pressure, the torque converter or clutch can slip, and the tachometer may have its own tolerance. Use the loaded radius of the tire for a better estimate.
Can I use it to choose a gear ratio?
Yes. Enter a candidate ratio and a road speed to see the engine speed, or solve for the ratio from the highest acceptable rpm. Check the result against the engine's power band and the tire load rating.
Does tire width matter?
Only through the rolling diameter. A wider tire with the same diameter turns at the same rpm. If the new tire has a different diameter, enter that diameter rather than the nominal wheel size.