Motorcycle Gear Ratio Calculator
Works out a motorcycle’s overall gearing and road speed at any rpm, and shows what a sprocket change does — the usual first modification on a sports bike.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
final drive = rear teeth ÷ front teeth overall = primary × gearbox × final drive speed = rpm ÷ overall × tyre circumference × 60
Engine speed is divided by three stages in series — primary, gearbox, chain — to reach the wheel, and the wheel covers its circumference every turn. Changing a sprocket changes the final drive, which multiplies through every gear equally.
What to watch out for
Sprocket changes, in practice:
- Minus one on the front ≈ plus three on the rear: about 6–7% shorter gearing. Quicker acceleration, lower top speed, higher revs at cruise.
- The speedometer lies afterwards on bikes that read from the gearbox rather than the wheel — by the same percentage.
- Chain length may need a link added or removed; the adjusters only cover a tooth or two.
Gearbox and primary ratios are in the workshop manual, and they are the part most people have to look up.
Frequently Asked Questions
What does changing the rear sprocket do?
Two more teeth on a 45-tooth rear shortens the gearing by about 4.4% in every gear — more acceleration, slightly lower top speed, about 4% more revs at any road speed. One tooth off the front does about the same as three on the rear.
How do I calculate motorcycle speed from rpm?
Divide rpm by the overall ratio (primary × gear × final drive) for wheel rpm, then multiply by the tyre circumference and 60. 8,000 rpm through a 5.7:1 overall ratio on a 1.96 m tyre is about 165 km/h.
Does a sprocket change affect the speedometer?
If the speedo takes its signal from the gearbox or front sprocket, yes — it will read high after shortening the gearing by the same percentage you changed. A front-wheel sensor is unaffected.