According to Power Masters Motorsports Academy, determining the ratio is a matter of counting teeth. Count the teeth on the driven, or wheel, sprocket, and divide that by the number of teeth on the drive, or transmission, sprocket. For example, suppose the larger wheel (or driven) sprocket has 16 teeth, and the smaller drive sprocket has eight teeth. Divide 16 by 8 and the result is 2. Similarly, suppose your driven sprocket has 20 teeth, and your drive sprocket has 10 teeth. Dividing 20 by 10 also yields 2. Ratios are expressed using two numbers; for example, the sprockets in both these examples have a 2-to-1 ratio. The ratio is not the size of the two sprockets; it is a comparison of the sizes of the two sprockets.
The interpretation of the ratio speed is where it starts to gets tricky. To find the speed of the driven sprocket, divide the speed of the drive sprocket by the ratio. Suppose your ratio is 2-to-1. This means for every time the driven sprocket makes one revolution, the drive sprocket has to make two revolutions. Suppose the drive sprocket is turning at 1,000 rpm. The speed of the driven sprocket is 500 rpm, since 1,000 divided by 2 is 500. If the ratio was 4-to-1, the drive sprocket would be turning at 1,000 rpm, but the driven sprocket would be turning at 250 rpm, since 1,000 divided by 4 equals 250.
To calculate the horsepower increase, multiply the ratio by the horsepower present at the drive sprocket. Remember, however, that you only have a finite amount of energy to experiment with. What you lose in one form, you gain in another. Suppose your engine puts out 10 horsepower. Feeding 10 horsepower into a 2-to-1 ratio means the speed of the driven sprocket is half that of the drive sprocket. However, the horsepower is doubled. The wheel is actually seeing 20 horsepower. If the ratio was 3-to-1, the horsepower would be 30, but the speed of the driven sprocket would be only one-third of the drive sprocket's speed. If the ratio was 4-to-1, the horsepower would be 40, but the speed of the driven sprocket would be only one-quarter of the drive sprocket.
According to the Power Masters Motor Sports Academy, the torque developed at the wheel is related to engine rpm and horsepower. Using a lower ratio, such as 2-to-1, means higher top speed, but with slower acceleration at low speeds. Using a higher ratio, such as 4-to-1, means faster acceleration off the line, but a slower top speed. Again, it's a trade-off, according to DIY Go Karts. DIY Go Karts has developed a calculator that accounts for a variety of factors, such as tire size, engine rpm and ratio. You enter all the data and the calculator determines your final top speed.