Although I recognize that theory, I have changed the entire top end on this Trooper including head, compllete intake throttle body, etc. The only thing on this Trooper that hasn't been changed is the short block, transmission and ECM.... When I first got it running I put an AMC head on it so I attributed the amazing gas mileage to that, but I pulled that head a while back and replaced it with a stock head, and the mileage didn't change. Due to some other problems I was having I installed a different intake that I had completely disassembled, cleaned, cherry picked parts from other Troopers, etc. and although it fixed my erratic idle and stalling problem, the fuel mileage stayed 21-22... So... either something's been done to the motor, or the auto trans gets better fuel mileage for some reason. I went to the federal EPA fuel mileage website and it shows the automoatic getting 1 MPG more than the 5 speed which I thought was pretty unusual, usually it's the other way around, but I've owned several 5 speeds and they've never gotten anyewhere near the kind of mileage this automatic gets.... I dunno