In gear drives, which measurement is used to calculate gear ratio by comparing two gears' sizes?

• A. Center distance
• B. Tooth count
• C. Module
• D. Pitch diameter

Answer: (D)

Gear ratio comes from comparing the sizes of the gears that mesh. When two gears engage, the tangential speed at the pitch line must match, so the relation ω1 * (d1/2) = ω2 * (d2/2) holds. This gives ω2/ω1 = d1/d2, meaning the speed ratio is the inverse of the pitch diameter ratio. The pitch diameter is the actual size measurement at the gear’s pitch circle, which is what governs how the gears mesh and how the speeds relate. Since pitch diameter and teeth count are linked (through the module), pitch diameter is the standard way to express and calculate the gear ratio from the gears’ sizes. Center distance describes spacing, not size; module describes tooth geometry but not the size used directly in the ratio calculation.