Motorcycle Gear Ratio

This calculator chains together three separate gear reductions -- primary drive, gearbox, and final drive (the sprocket-to-sprocket chain ratio) -- into one Overall Ratio, then converts that ratio plus tire circumference into road speed at a given engine RPM. Final Drive Ratio is simply Rear Sprocket Teeth divided by Front Sprocket Teeth, and because Overall Ratio is a straight product of Primary Drive Ratio, Gearbox Ratio and Final Drive Ratio, Speed is inversely proportional to Overall Ratio as a whole. That means Rear Sprocket Teeth, Primary Drive Ratio and Gearbox Ratio all push the same way -- raise any of them and Speed at a fixed RPM falls -- while Front Sprocket Teeth is the one input that sits in the numerator: a larger front sprocket lowers Overall Ratio and raises Speed. One tooth on the front is worth roughly three on the rear precisely because the front count is the smaller number being divided into. Rear +1 Tooth: Ratio Change and Rear -1 Tooth: Ratio Change convert that same sensitivity into a practical sprocket-swap planning figure, showing the percentage change in Final Drive Ratio from the single most common gearing adjustment riders make.

Read the sign carefully: because Speed is inversely proportional to the ratio, a POSITIVE ratio change means shorter gearing -- more acceleration and a LOWER speed at any fixed RPM. At the default 15/45 gearing, +1 rear tooth is a +2.2% ratio change, which works out to about 2.2% less speed at the same engine RPM. RPM per km/h is the inverse relationship flipped around: how many engine RPM one km/h of road speed costs at the current gearing, useful for reading a tachometer against a speedometer at a glance. Tire Circumference is worked out from the sidewall code: for 180/55-17, sidewall = 180 x 0.55 = 99 mm, diameter = 17 x 25.4 + 2 x 99 = 630 mm, circumference = pi x 630 is about 1,979 mm; subtract roughly 1% for rolling circumference under load to get about 1,960 mm, the default used here.