Tool
Sprocket Calculator
Enter two sprocket pairs. Pair A is the baseline. Pair B is the set you are considering. The calculator reports each rear-over-front ratio, the percent change, and the MPH difference at one RPM if you also enter primary, one gearbox ratio, and a tire.
Pair A / pair B
—
Pair B: —
Change in final drive:
When to use this calculator
Use it when you are holding two receipts and want the percent difference before you pull a countershaft nut. The Motorcycle Gearing Calculator is the single-pair view; this page is the side-by-side view.
The MPH line uses the same formula as speed in gear, but only for one gearbox ratio. It answers a single question: at this RPM, how much road speed do you give up or gain?
Percent change in ratio and percent change in MPH have opposite signs. Shorter gearing raises the ratio and lowers MPH at the same RPM.
Zero teeth stop the comparison with a field error.
Worked pairs
Pair B is 6.67 percent shorter. At the same RPM, road speed is 6.25 percent lower, because speed scales with the inverse of final drive.
Two extra rear teeth are 4.44 percent shorter. The MPH change is smaller than a one-tooth front change on a 16-tooth countershaft.
If both pairs match, the percent change is zero. That is the check that the comparison is using rear ÷ front on both sides.
Technical questions
Why does MPH fall when the ratio rises?
Overall reduction includes final drive. A larger reduction means fewer wheel turns per engine turn, so the same RPM produces a lower road speed.
Do I have to enter a tire?
The two ratios and the percent change do not need a tire. The MPH sentence needs a tire, a primary ratio, one gearbox ratio, and an RPM.
Is chain size part of the ratio?
No. 520, 525, and 530 change width and strength, not the tooth-count ratio. Chain length is a separate tool.