How the gear ratio RPM calculator works
Engine RPM reaches the wheels through two reductions: the gearbox ratio for whichever gear is selected, and the final drive ratio in the differential. Multiplying the two gives the combined ratio between the engine and the wheels, and dividing engine RPM by that combined ratio gives wheel RPM.
Gear ratio to RPM formula
combined ratio = gear ratio × final drive ratio
wheel RPM = engine RPM ÷ combined ratio
speed in mph = wheel RPM × tire circumference in inches × 60 ÷ 63,360
Example
At 6,000 engine RPM, with a 3.36 gear ratio, a 3.73 final drive, and a 26 inch tire, the combined ratio is 12.53:1.
That works out to roughly 479 wheel RPM, or about 61.6 mph.
This page works as a way to calculate rpm gear ratio combinations for a given engine speed, and as a gear ratio speed calculator once a tire diameter is added, since the same combined ratio determines both wheel RPM and road speed.
Frequently asked questions
What is a combined gear ratio?
It is the gear ratio multiplied by the final drive ratio. This single number describes the total reduction between the engine and the wheels for that gear.
Does tire diameter change the RPM result?
No. Wheel RPM depends only on engine RPM and the combined ratio. Tire diameter is only needed to convert wheel RPM into a road speed.
Can I use this for a motorcycle or a vehicle without a separate final drive?
Yes. If there is no separate final drive stage, enter 1 for the final drive ratio and the gearbox ratio alone will be used as the combined ratio.
Is this accurate for real world driving?
It gives a theoretical figure based on the ratios and tire size entered. Real speed can differ slightly due to tire growth at speed, wheel slip, and manufacturing tolerances in the actual ratios.