Tools › Gear Ratio & Speed
Drivetrain & Gearing
Gear Ratio & Speed Calculator
See road speed in every gear at any RPM, your cruise RPM at motorway speed, and the theoretical top speed each gear allows, straight from your tyre size, final drive and gearbox ratios. Ideal for diff swaps, re-gearing and tyre changes.
Fill in the gears your car has, and clear any you don't need. The highest (numerically lowest) gear is treated as top.
Gears shown schematically for very low gears (the spin ratio is capped to stay readable). The figures above are exact.
How gear ratios turn RPM into road speed
Your engine speed reaches the road through two reductions: the gearbox ratio for the gear you're in, then the final drive (the differential). Multiply them and you get the overall ratio: how many times the engine turns for one turn of the wheels.
From there it's simple: wheel RPM = engine RPM ÷ overall ratio, and road speed = wheel RPM × rolling circumference. The tyre's rolling circumference comes from its size, which is why a tyre change quietly re-gears the whole car.
Why this matters for swaps and re-gearing
- Diff / final-drive swap: a numerically higher final drive (a "shorter" diff) sharpens acceleration but raises cruise RPM and brings the redline up sooner in each gear.
- Taller top gear or overdrive: drops motorway RPM for quieter, more economical cruising, at the cost of in-gear punch.
- Bigger or smaller tyres: larger tyres effectively lengthen every gear (lower RPM, speedo under-reads); smaller tyres shorten them.
The gearing ladder chart plots road speed against RPM for every gear at once, so you can see exactly where gears overlap and what RPM you'll sit at for any cruising speed.
Reading the cruise RPM figure
Cruise RPM is the engine speed in top gear at your chosen road speed. Lower is generally quieter and more economical on the motorway, but go too tall and the car becomes strained on hills and slow to respond without a downshift. Most relaxed petrol cars sit around 2,000–2,800 rpm at 70 mph.
Frequently asked questions
How do I work out speed from RPM and gearing?
Wheel RPM = engine RPM ÷ overall ratio (gear × final drive); road speed = wheel RPM × tyre rolling circumference. This tool does it for every gear at once.
What is the overall (total) gear ratio?
The selected gearbox ratio multiplied by the final drive. It's how many engine turns equal one wheel turn. Higher means more acceleration and lower top speed in that gear.
Does a higher final drive make the car faster?
It improves acceleration and in-gear response but raises cruising RPM and usually lowers realistic top speed, since each gear hits the redline at a lower road speed.
How do bigger tyres affect gearing?
Larger tyres effectively make every gear taller, giving lower RPM at a given speed and an under-reading speedometer. Smaller tyres do the reverse. The calculator accounts for tyre size so you can compare fitments.