In a gear drive, which dimension is used to compare the gear ratio?

• A. Number of teeth
• B. Pitch diameter
• C. Module
• D. Center distance

Answer: (B)

The key idea is that gear ratio is about how the sizes of meshing gears relate to their speeds. At the point where the teeth engage, the tangential (pitch-line) velocity must be the same for both gears. If d1 and d2 are the pitch diameters of the two gears, then the speeds satisfy ω1·(d1/2) = ω2·(d2/2). This gives the speed ratio ω1/ω2 = d2/d1. So the ratio of the gears’ speeds is directly determined by the ratio of their pitch diameters.

Because the pitch diameter is tied to the number of teeth through the relation d = m·z (where m is the module and z is the number of teeth), the pitch diameter provides a single, measurable dimension that reflects the gear size and contact geometry. This makes it a natural basis for comparing gear ratios: gears with larger pitch diameters will rotate more slowly, and the ratio of those diameters matches the gear ratio.

The center distance is related to the sum of the radii and changes with gear sizes, but it is not a direct measure of the ratio itself. The module is a design parameter, not a dimension you compare to determine the ratio, and the number of teeth is a discrete count rather than a physical dimension—pitch diameter remains the practical dimension to compare gear ratios.