Changing your bike’s gear ratio is the easiest way to change how the bike performs. If you feel like your bike is sluggish off the line, just bump up your gear ratio. If you feel like your bike is too quick off the line and your bike tops out at 45 miles an hour, drop your gear ratio.
Gear ratio is calculated by the tooth counts of your front and rear gears. To figure out your current gear ration, divide the rear gear by the front gear.
I have a ’78 Honda CB400 Hawk II. The rear gear has 37 teeth and the front has 16 teeth. Dividing 37 by 16 gives me a gear ratio of 2.3125.
I feel like the bike is too sluggish off the line and I enjoy ripping up dirt roads and city riding, so I’m going to bump up the gear ratio.
To do that I could increase the rear sprocket to a 38-tooth sprocket. That would increase my gear ratio from 2.3125 to 2.3750. That’s a 2.7 percent increase.
But I’m going all out with a 15-tooth front sprocket. That will leave me with a 2.4667 gear ratio. That’s a 6.6 percent increase. That might prove to be too much, and if so, that will suck, but I’ll probably be popping wheelies all day long.
Just remember that when adjusting your gear ratio, changes to the rear sprocket have less impact than the front. That’s why a one-tooth increase in the rear sprocket is only a 2.7 percent increase, while a one-tooth decrease in the front sprocket is a 6.6 percent increase.
I purchased both sprockets from JC Whitney.