Blower drive question

Sat. I ran the "High Times" Crosley Dragster. Best ET yet! (18.10) Thanx to a new Stromberg 97 from the Hot Rod Co.! Shutting down early due to oil leak. (I thought it was gas!) My question is :what is my blower drive over drive%. I have a 5" crank pulkley & a 4 1/4" blower pulley. Does anyone have the formula to figure this out? Thanx~!! Arnie

 

Approx. 17.5% if my math is correct, however; it usually goes by tooth count on the pulleys.

 

5 divide by 4.25 = 1.176 so it's about 18" overdriven. But that's probably not exact, because you probably measured the outside diameter instead of the pitch diameter. It should be pretty close though.

 

Thanx! I thought there would be an easy way!