final drive ratio.
Divide the number of teeth in the rear sprocket by the number of teeth in the front to get the final drive ratio. Example: 35 / 15 = 2.33:1
If you want to change your gearing, you can calculate the impact of the change before buying a new sprocket.
Example: Your bike is cruising at 60 mph / 4,000 rpm with a 2.33:1 final drive ratio – if you want more acceleration, replace the 35-tooth sprocket with a 37-tooth sprocket and keep it in the front 15 teeth changes the gear ratio to 2.47:1 (37 / 15 = 2.47:1). Divide your cruise RPM by 2.33 and multiply by 2.47. Using the example above, 4,000 rpm becomes 4,240 rpm at 60 mph. You get more acceleration, but at the expense of
Changing the sprocket diameter can be problematic - the front sprocket clearance limits how far you can go, and you want to avoid going too small as tight turns will wear out the chain prematurely. Also, if you are too small in the front and/or rear, you may have issues with the chain flapping the swingarm. You can't always see this when the bike is stationary, the combination of swingarm movement over bumps and acceleration or deceleration causes the chain to mark on the top or bottom of the swingarm.
Changing sprocket size may affect chain length. Since each sprocket has about 1/2 of the chain touching it, each tooth added to the stock size will require about 1/2 additional chain links. By moving the rear axle slightly forward and keeping the stock chain length, you can usually increase the total number of teeth by one or two teeth, but if you start changing the sprocket size by more than one or two teeth, you will need to increase the chain length. Four teeth up will require 2 links of a longer chain.
