How the gear ratio is calculated
Final drive ratio = rear sprocket teeth ÷ front sprocket teeth. A 14-tooth front and 42-tooth rear is 42 ÷ 14 = 3.0 : 1: the countershaft turns three times for every turn of the rear wheel.
| Change | Ratio | Effect |
|---|---|---|
| Smaller front, or bigger rear | Higher number | “Lower” gearing: harder acceleration, wheelies easier, less top speed, more RPM at cruise |
| Bigger front, or smaller rear | Lower number | “Taller” gearing: calmer pull, more top speed if the engine has the power, lower RPM at cruise |
Front or rear: which should you change?
One tooth on the front sprocket changes the ratio far more than one tooth on the rear – on a 14/42 setup, going to a 13 front is the same as going about 3 teeth bigger on the rear. The front is cheaper and quicker to swap, so it’s the usual first try. The rear gives finer steps when you only want a small change.
Don’t go too small on the front: very small sprockets wear chains faster and can hit the case saver or swingarm slider. Most 110–125cc pit bikes run 14 or 15 teeth; going below 13 is rarely a good idea.