It's a parts mismatch. The cam's too big, and the carb is WAY too big. Event he Comp catalog says a minimum of 9.5:1 and you're below that. On the carb - Compared apples to apples with an older carb you have more than 880cfm with that carb. The dyno shows it's a pig - no offesnse but 390/390 out of a 340 is terrible given the work done. Hence the inability to be efficient and poor mileage. Stick a cam in it that has 225 - like the Comp XR274HR10 - and put on the Avenger 570 and you'll pick up a shitload more torque and horsepower, and mileage.