How to Use This Tool
Enter your tyre size, final drive and the gear ratios. Everything else is one equation applied to each gear in turn.
The chain
- Road speed to wheel speed. Divide by the tyre's rolling circumference. A 225/45R17 is 634.3 mm across, so 1.9927 m around; 100 km/h is 1,667 m per minute, which is 836 wheel revolutions per minute.
- Wheel speed to gearbox output. Multiply by the final drive — 836 × 3.90 = 3,262.
- Through the gear. Multiply by the gear ratio. In a 0.80 top gear that is 2,610 rpm at 100 km/h.
Written as one line: rpm = speed ÷ circumference × gear × final.
Circumference is the only term that divides, which is the whole reason a tyre change is a gearing change.
Tyre size is a gear ratio you did not choose
Fit a tyre 3% larger and every one of those numbers shifts by 3%: cruising revs drop 3%, the speed reached at the redline in each gear rises 3%, and acceleration softens slightly because the effective final drive has been lowered.
It works the other way too. A slightly smaller tyre is a cheap way to shorten gearing, which is why winter wheels a size down often feel a touch livelier.
The speedometer is affected by exactly the same percentage, in the direction that makes it read optimistically low with a taller tyre.
Reading the gear table
Two columns are worth attention:
- Speed at the redline is a gearing figure, not a performance one. A gear that reaches 249 km/h at the redline tells you the car cannot use that gear fully — it will run out of power against air resistance long before it runs out of revs. That is normal for a tall top gear, which exists for economy and noise rather than speed.
- Revs after the upshift is where the driving feel lives. Shifting at the redline drops the engine to (next gear ÷ current gear) × redline. Closely spaced gears land back near peak power; a wide gap drops the engine below it, which is why one badly spaced ratio can make a whole gearbox feel wrong.
What is left out
This is kinematics: it maps engine speed to road speed and nothing more. It cannot tell you whether the engine can pull a gear, because that needs power against aerodynamic drag, rolling resistance, weight and gradient.
Tyres also grow slightly at speed and deform under load, so real rolling circumference is a little different from the geometric figure — enough to matter to a race engineer and not to anyone else. Automatics with a torque converter slip at low speeds and lock up above them, so the calculation only applies once locked.