Gear ratio explained. Gear ratio is the ratio of the number of revolutions of one gear compared to the other when the gears are connected by a chain. An example of a gear ratio is a 10-tooth gear and a 20-tooth second gear with a gear ratio of 2 (20/10=2). To calculate this ratio, divide the number of teeth on the rear sprocket by the number of teeth on the front sprocket, and the resulting number is your gear ratio.
Why do you need to know your gear ratio? It could be that your bike isn’t running the way you want, and a gear ratio change could help improve performance.
GEAR DOWN.  An example of this is that the bike is hard to start in first gear, and the clutch needs more use than you would like. You have enough high-end speed but need slower speed control. The answer may be to slow down the bike. You can do this by adding more teeth on the rear sprocket or reducing the number of teeth on the front sprocket.
GEAR UP.  Conversely, it could be that the bike has a lot of power but hits the rev limiter in top gear, and you want to rev down at a specific speed. To do this, you need to equip your bike by reducing the number of teeth on the rear sprocket or increasing the number of teeth on the front sprocket.
Diameter of gear. Each tooth added or removed from the sprocket reduces the diameter of the sprocket by a given amount. For simplicity, you can consider a 5mm change in diameter for each additional or missing tooth. This will help us understand the space requirements for gear ratio changes.