Gear Ratio Calculator
Works out a gear ratio from tooth counts and multiplies it through the final drive. The overall ratio is the number that actually determines how engine speed relates to wheel speed.
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The formula
gear ratio = driven teeth ÷ driving teeth overall ratio = gear ratio × final drive
A ratio above 1 is a reduction: the output turns slower than the input and torque rises by the same factor. Below 1 is an overdrive — faster output, less torque.
What to watch out for
What the ratio buys and what it costs:
- Higher numerical ratio = more torque, less speed. A 4.10 diff accelerates harder than a 3.55 and cruises at higher revs for the same road speed.
- Torque and speed trade exactly. Ignoring losses, a 4:1 reduction quadruples torque and quarters speed. There is no free ratio.
- Overall ratio is what matters. Gearbox ratio alone tells you little — first gear at 3.6:1 through a 3.9 diff is 14:1 at the wheels.
- Changing the diff changes every gear. That is why it is the usual lever for shifting a car's whole character.
Frequently Asked Questions
How do I calculate gear ratio?
Divide the driven gear's teeth by the driving gear's teeth. 72 driven over 20 driving is 3.6:1 — the input turns 3.6 times per output turn.
What does a higher gear ratio do?
More acceleration, lower top speed, higher cruising revs. A numerically higher ratio multiplies torque more but reaches a given road speed at higher engine speed.
What is the overall ratio?
Gearbox ratio multiplied by final drive. It is what connects engine RPM to wheel RPM, and the number to use when working out road speed.
What is an overdrive gear?
Any ratio below 1:1, where the output turns faster than the input. Top gears are usually overdriven so cruising revs stay low.