Nominal Vs. Exact Gear Ratio

Exact Gear Ratio

• Exact gear ratio is found by dividing the number of teeth on the powered gear by the number of teeth on the gear. It can also be the ratio of their diameters. If a powered gear has 10 teeth and turns a gear that has five teeth, the gear ratio is 10-to-5 reduced to 2-to-1.

Nominal Gear Ratio

• Nominal gear ratio can be used to estimate the gear ratio of an entire set of gears. Nominal gear ratio is a ratio to the power or speed of the powered gear to the output gear. If the powered gear turns at 50 RPM and the output gear at 20 RPM, the nominal gear ratio is 5-to-2.

Gear Ratio Usage

• Gear ratios can determine the speed of rotations per minute that the driven gears make per turns of the driven gear. "Automotive Technology: a Systems Approach, Volume 2" by Jack Erjavec states "a gear ratio also expresses the amount of torque multiplication between two gears."