If a pulley system has a driver pulley with a pitch diameter of 4 in. and a driven pulley with a pitch diameter of 16 in. the pulley ratio is __.

• A. 1:4
• B. 4;1
• C. 1:16
• D. 16;4

Answer: (B)

Pulley ratio is determined by the relationship between the driven and driver diameters. With the driven pulley at 16 inches and the driver at 4 inches, the ratio is 16 divided by 4, which equals 4. This is written as 4:1, meaning the driven pulley is four times larger in diameter than the driver. Consequently, the driver must rotate four times for the driven to complete one turn, so the output speed is one quarter of the input speed, and the torque at the driven side increases by about four times (ignoring losses). Writing the ratio as 1:4 would describe the opposite relationship, which doesn’t match the given sizes. If you see 16:4, that’s the same ratio as 4:1 when simplified.