How to Find the Gear Ratio

The gear ratio is a numerical ratio that describes the relationship between two gears. It is a simple, objective way to define the different ratios at which two gears are turning relative to one another. For example, a large gear that takes 10 times as long to make a full rotation as a smaller gear would be said to have a 10:1 ratio with the smaller gear.

Things You'll Need

• 2 interlocking gears

Instructions

1. Count the number of teeth in the larger gear, also called the idler gear. For the sake of example, let's say the idler gear has 30 teeth.

2. Count the number of teeth on the smaller gear, also called the pinion. For the sake of example, let's say that the pinion has 20 teeth.

3. Make a fraction with the idler gear's number of teeth on top, and the pinion's number of teeth on bottom. In our example, the fraction is 30/20.

4. Reduce the fraction to its simplest form. The simplest form of 30/20 is 3/2, or 1.5.

5. The gear ratio is determined by taking this number, 1.5, and putting it over 1. 1.5:1 is the gear ratio from our example. This means that the smaller pinion gear must turn 1.5 times for every 1 rotation that the larger idler gear makes.

Tips & Warnings

• Although you can estimate the gear ratio by comparing the size of the two different gears, there is no way to get a precise gear ratio without counting gear teeth.