This is a numerical ratio of a series of gears in relation to each other, based on the number of turns of the input shaft, compared to turns of the output shaft. Gear ratios are determined by the number of teeth on each gear (and therefore the size of each gear). For instance, a gear with 36 teeth meshed with a gear with 12 teeth gives a 36/12 or 3/1 ratio. This is usually expressed as 3.00:1.