The pulley ratio is determined by using the _______________ of the pulleys, which is not the same as the outer diameter.

• A. Outer Diameter
• B. Width
• C. Ring Size
• D. Pitch Diameter

Answer: (D)

The key factor is the belt’s pitch line. The pulley ratio comes from the pitch diameter—the diameter of the circle traced by the belt’s pitch points where the belt actually engages the pulley. This is the effective diameter that determines how much belt length passes per rotation, so it sets the speed relationship between the pulleys. Outer diameter includes tooth height and belt thickness and doesn’t reflect how the belt engages the pulley, so it isn’t used to determine the ratio. Width and ring size aren’t related to how much belt movement occurs per revolution either. In short, the pitch diameter is the correct measure because it matches the belt’s contact point and defines the velocity ratio between pulleys.