A gear-reduction ratio is calculated directly from the number of teeth on each gear. The number of teeth is a simple value to obtain and that is all you need to complete this computation. After you calculate this ratio, you can use it for any other computation you -- need such as speed or torque.
Count or obtain the number of teeth on each gear. Calculate the gear reduction ratio by dividing the second gear's number of teeth by the first gear's number.
Reduce the resulting fraction to make it easier to read. For instance 100/75 becomes 4/3.
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Reduce your calculated ratio further to an X:1 format, which is most commonly used by calculators and equations. For example, a 4/3 ratio becomes 1.33:1.