Adjusting the bypass affects fuel mileage more than it does performance (it's tiny if actually there at all). Adjusted so that only a slight tip in of the throttle gets the boost coming on puts a pretty good dent in cruising mileage. On a cross country trip from WA to MI and back last year I had mine set pretty aggressive going to MI and noticed my mileage took a big hit. Wondered if it was something else so for the trip back I adjusted it so that it was a lot less agressive and mileage went back up where it had been. Avg mileage on the way was 23 compared to a previous trip where I got 29 over the whole trip. Set the bypass up like it was stock and my mileage went back up to that number.

RC