Most all the people that I have talked to that have chipped their engine or add headers etc have never seen any real MPG increase. All have reported improved performance and less shifting on hills. My personal experience with mods like headers, FF mufflers, CAI, advancing the timing all improved performance as measured by 40 -60 MPH times. Times went from 21 sec down to 12 secs but MPG stayed the same at 8.5 MPG. The only difference I saw was when I installed an old vacuum gage I had laying around in my garage, I saw my MPG go up to 8.7. By watching the gage I knew when the transmission was going to downshift so I would reduce the throttle keeping the vacuum above that point. You are right you don't want your transmission to downshift all the time I find my best milage is when I keep the rpms just above max torque which is different for each engine. I must have posted the same time as Gdetrailer.
