I dont know for sure how much HP I gained with the smaller pulley but I went from 6~7 psi to 10~11psi. I ve had it on for about 2 1/2 years going on 3, 52k miles. With that I have a Flowmaster and FIPK. Put a boost gauge on before doing this mod so you have something to measure the increase by other than your butt. I recommend the pulley ASP has to remove the pulley. Some say they dont recommend this mod becuase of detonation problems they ve experienced. I personally havent encountered this problem. Gas mileage hasnt increased or decreased, it all depends on how heavy my foot is that day. I m glad I did this mod as it is the most noticebale increase in power.