Sprocket & Gearing Calculator
What a sprocket change does to your revs, your speed and your speedometer — calibrated from one point you already know.
A point you already know
Any gear will do, as long as both numbers came from the same moment. Everything after the countershaft scales with the final drive, so this pair is all the calibration needed — no primary or gearbox ratios required.
Nearby options
| Sprockets | Ratio | Change | RPM at your speed |
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Tire size and chain length
Why the front sprocket does so much
Final drive is rear teeth divided by front teeth, so a tooth off a 16-tooth front is a six per cent change while a tooth on a 45-tooth rear is barely two. One front tooth is worth about three on the rear — which makes it the cheapest change available and, frequently, far more change than anyone wanted.
Shorter gearing does not add power. It trades top speed for acceleration and makes the bike feel livelier, and feeling livelier is not the same as being quicker anywhere that matters.
The speedometer will lie to you
A speedometer driven from the gearbox has no idea the sprockets changed — it counts countershaft revolutions and multiplies by whatever ratio the factory assumed. Gear down and it over-reads: you are going slower than it claims, which is the harmless direction for your licence and the irritating one for your fuel economy figures. Bikes that take their speed from a front wheel or ABS sensor are unaffected, so it is worth knowing which yours does before you conclude the bike got slower.
Things worth knowing
- One tooth on the front is worth about three on the rear, because the front sprocket has so few teeth to start with. It is the cheapest and most dramatic change you can make, and often too dramatic.
- Shorter gearing does not add power. It trades top speed for acceleration and makes the bike feel livelier, which is not the same thing as being faster anywhere that matters.
- A speedometer driven from the gearbox has no idea you changed the sprockets. Gear down and it over-reads — you are going slower than it says, which is the harmless direction for your licence and the annoying one for your trip computer.
- Tire size changes gearing too. On a typical 25-inch rear wheel, an inch of extra diameter is about 4% taller final drive — more than a tooth on the rear sprocket, achieved without touching either one.
- Replace sprockets and chain together. A worn chain wears new sprockets quickly, and a worn sprocket ruins a new chain — mixing them wastes whichever part was new.
How to use
- Enter your current sprockets and the pair you are considering.
- Give one road speed and the rpm you see at it — any gear, same moment.
- Read the change, the new rpm and speed, and what the speedometer will show.
Frequently asked questions
How much difference does one tooth make?
On the front, about six per cent — a tooth off a 16-tooth sprocket is a big change. On the rear, barely two. One front tooth is worth roughly three on the rear, which makes it the cheapest change available and often far more than anyone wanted.
Is shorter gearing faster?
Not in any meaningful sense. It trades top speed for acceleration and makes the bike feel livelier, which is not the same as being quicker. It adds no power at all.
Do I need my gearbox ratios?
No. Everything after the countershaft scales with the final drive, so one speed-and-rpm pair from any gear is enough to calibrate the whole thing exactly.
Will my speedometer still be right?
If it is driven from the gearbox, no. It counts countershaft revolutions and assumes the factory ratio, so gearing down makes it over-read — you are going slower than it says. Bikes that take speed from a front wheel or ABS sensor are unaffected.
Does tire size change gearing?
Yes, and by more than people expect. On a typical 25-inch rear wheel, an extra inch of diameter is about four per cent taller final drive — more than a tooth on the rear sprocket, without touching either one.
How do I work out chain length?
Two times the centre distance divided by pitch, plus half the total teeth, plus a correction for the difference in sprocket sizes. Always round up to an even number: a chain has to close on itself, and an odd count needs an offset link you do not want on a motorcycle.
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