Gear Speed Calculator
Enter an engine RPM, your rear tyre size, and the driveline ratios to see the road speed in every gear — useful for spotting gearing gaps and checking a sprocket change.
From crankshaft to contact patch
Every road speed here traces the same path: the engine drives the clutch through the primary reduction, the gearbox applies the selected ratio, and the chain applies the final drive. Multiply those three together and you know how many times the engine turns for one turn of the rear wheel; divide engine RPM by that and multiply by tyre circumference to get speed.
Change the final drive with the Chain & Sprocket Ratio Calculatorand every gear’s speed shifts; fit a taller or shorter rear tyre and the Tire Size & Speedo Error Calculator shows how the circumference — and these speeds — change again.
Frequently Asked Questions
How does the calculator turn RPM into speed?
It divides engine RPM by the product of the primary reduction, the selected gearbox ratio, and the final drive ratio to get rear-wheel RPM, then multiplies by the tyre's rolling circumference. Circumference comes from the rear tyre size you enter, so the speed reflects your actual wheel and gearing, not a generic assumption.
Where do I find the primary and gearbox ratios?
They are in your bike's service manual or specification sheet, usually listed as 'primary reduction ratio' and a set of gear ratios from first to top. The final drive ratio is simply your rear sprocket teeth divided by your front sprocket teeth — the Chain & Sprocket Ratio Calculator works it out for you.
Why is my indicated speed a little higher than this figure?
Two reasons. Most factory speedometers deliberately read a few percent optimistic to stay legal, and the calculation uses the tyre's nominal unloaded circumference, whereas a loaded, rolling tyre is slightly smaller. Expect the true speed to sit a few percent below the number on your dash and close to this estimate.
Can I use it to check for a gap between gears?
Yes — that is one of its best uses. Read the speed at redline in each gear and at the RPM you would drop to in the next gear. A big drop means a tall gap where the engine falls off its power; a small drop means closely stacked ratios that keep it on the boil. Sprocket changes shift every gear's speed together.
Educational estimate from nominal tyre circumference. Tyre slip, squat, and speedo optimism mean true speed runs a few percent from the calculated figure.