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Just for reference, the circumference of a 3” pulley is pi x diameter or 3.14 x 3” or 9.42” and the 2.75” pulley is 3.14 x 2.75 or 8.64”.
Say you were going to string a rope of almost infinite distance apart like say from our solar system to the other side of the universe, and then you decided to lift this perfectly straight rope by the distance of the 3” pulley then you swapped to a 2.75” pulley. The difference in length of the rope would be practically zero, but not zero.
Next, say you wrapped the pulleys completely with the rope, so the incoming and outgoing lines of the rope touch. Then the difference in distance around the pulleys is the circumference or 0.78”.
The routing diagram of the belt around the supercharger pulley to the other pulleys is neither so flat as to approximate infinity, nor is it so extreme as to approximate circumferential. This means that the new belt length will be between 0” and 0.78” shorter, yet considering the diagram angles look to wrap about three quarters of the pulley then it’ll be close to 0.6” shorter. Therefore, if your belt was perfectly taught previously with the 3” pulley then buying a belt that is around 0.6” shorter would be ideal. If it was already loose then Bill’s suggestion of 1.5-2” could be fine.
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