Gear Ratio & RPM Calculator

Engine power reaches the road through the gearbox and the final drive, each of which reduces rotational speed. Dividing engine RPM by the product of the gear ratio and final-drive ratio gives wheel RPM; multiplying wheel RPM by the tyre’s rolling circumference gives road speed.

Worked example: with a 3.6 gear ratio, 4.1 final drive and a 225/45 R17 tyre (circumference ≈ 1.99 m), 3,000 engine RPM gives wheel RPM = 3,000 ÷ (3.6 × 4.1) = 203.3, and speed ≈ 203.3 × 1.99 × 60 ÷ 1,000 = 24.3 km/h. Doubling the engine RPM doubles the speed.

These are theoretical estimates assuming no tyre slip and a nominal (unloaded) tyre circumference. The real rolling circumference is slightly smaller because the contact patch flattens under load, and slip increases with torque. Drag, gearing inefficiency and engine power limits all reduce actual speed.

A speed calculated at the engine’s redline is not the car’s achievable top speed — it is simply the theoretical speed at that RPM in that gear. Top speed depends on whether the engine can produce enough power to overcome aerodynamic drag.