Gear Ratio Calculator
Engine speed at road speed, through the gearbox and the tyres.
Work out Gear Ratio. Engine speed at road speed, through the gearbox and the tyres. Free, with no account and nothing to install.
At 100 km/h
2,433 rpm
41.1 km/h per 1,000 rpm · overall ratio 2.907:1
The overall ratio is the gearbox ratio times the final drive. Tyre circumference then converts wheel revolutions into distance, which is why the tyre is part of the gearing whether or not anyone thinks of it that way. Fitting taller tyres lengthens the effective gearing: cruising revs drop, motorway refinement improves, and acceleration blunts. Shorter tyres do the reverse. A shorter final drive has exactly the same effect as shorter tyres and is the cheaper change on cars where the differential is accessible. Cruising rpm is what determines motorway fuel consumption more than almost anything else, which is why manufacturers have added ratios steadily — a modern eight-speed automatic exists mostly to keep the engine near its efficiency island at every road speed.
How the Gear Ratio Calculator works
Engine speed at a road speed through the gearbox ratio, the final drive and the tyre. The tyre is part of the gearing whether or not anyone thinks of it that way, which is why fitting taller tyres lowers cruising revs and blunts acceleration.
Also known as: rpm at 70 mph · km per hour per 1000 rpm · final drive ratio effect · will taller tyres lower my revs
Frequently asked questions
How do I calculate rpm at a given speed?
Wheel revolutions per minute times the overall ratio, which is the gearbox ratio times the final drive. Tyre circumference converts road speed into wheel revolutions, so all three are needed.
What does the final drive ratio do?
It multiplies every gear. A shorter final drive — a higher number — improves acceleration in every gear and raises motorway revs and consumption. A longer one does the reverse.
Do bigger tyres change my gearing?
Effectively yes. A taller tyre covers more ground per revolution, so cruising revs drop and acceleration softens. It is exactly equivalent to fitting a longer final drive, and much easier to do by accident.
What rpm should I cruise at?
Whatever keeps the engine in its efficient range, usually 1,500 to 2,500 for a modern petrol engine at motorway speed. Cruising rpm drives motorway fuel consumption more than almost anything else, which is why gearboxes have grown from four ratios to eight.
Why do cars have so many gears now?
To keep the engine near its efficiency island at every road speed. An eight-speed automatic is not about performance so much as about never being far from the most economical operating point.
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