Smaller water pump pulley

famous bob said

Any one know how to figure the ratio of pulleys ? same size is 1 to one , going smaller on my water pump---------

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Divide one diameter by the other. For instance, 7 inches divided by 5 inches is a 1.4 ratio. That means for every turn of the 7 inch pulley, the 5 inch one turns 1.4 times.