Calculating the speed ratio in a pulley or gear system allows the determination of final pulley speed and work accomplished. The ratio explains the speed of the motor-driven pulley as it relates to the object pulley, which is rotated by the motor-driven pulley. The ratio is typically offered in the format "3:1," which means the first pulley travels three times faster than the second pulley. Another way to say this is the second pulley rotated one full turn for every three rotations of the first pulley.

Measure the diameter of the motor-driven pulley.

Measure the diameter of the object pulley, which is the one attached to the motor-driven pulley.

Divide the diameter of the motor-driven pulley by the diameter of the object pulley. As an example, a 600mm motor-driven pulley diameter is divided by the 200mm object pulley diameter, which equals three. This tells you the speed, measured in revolutions per minute, of the motor-driven pulley is three times that of the object pulley. Therefore, the ratio would be expressed as 3:1.