Not sure what diameter the Moroso is, only that it says a 30% reduction in pulley speed, IIRC. My brain hasn't shifted into calculator mode to reverse-engineer the diameter.
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Reduction will be directly proportional to diameter.
Circumference, not diameter. 70% of 6.25 is only about 4.3...
And 70% of circumference still results in 4.3" diameter. Pulley speed, circumference and diameter are all proportional.
So either Moroso made an error, or their pulley is 4.3, or they started with a different pulley diameter for the claimed 70%. FWIW, the crank pulleys I have here range from 6.5 to 7.25.
