A nice jackshaft feature is that the 2 pulleys need not be the same size or shape.
Depending on what you have for the crank and blower, you may need more or less than the ratio of the 2 diameters: make the input side (crank driven) larger or smaller than the output (drives blower) side, and multiply that ratio by the existing.
E.g., crank is 9", blower is 6" (1.5:1), but you want 2.5:1.
Use a 5" on the drive from the crank and a 3" on the drive to the blower (1.67:1).
1.5 (existing ratio) × 1.67 (jackshaft ratio) = 2.5:1.
Sometimes you have too much ratio: the blower pulley is 3", giving 3:1. Slow this down to 2:1 by using a 4" on the driven side and 6" on the driver side: 4:6 = .67:1.
3.0 (existing ratio) × .67 (jackshaft ratio) = 2:1
In addition, it may permit you to save work in 2 ways:
1. use an existing multi V-belt pulley on the crank
2. use an existing serpentine (multi-rib flat belt) pulley on the blower
How? The jackshaft has a dual V pulley on the input side (common automotive or industrial part), and a serpentine on the driven side.
