Hello, I am in need of some advise since no one locally can help me. I have a 2003 Chevy 2500HD, ext cab, 6.0L gas, 4x4, KN cold air intake,75k miles and a custom tuned ECM. This truck is pretty much a dog. It will get 12 mpg normal driving and 9 pulling a 7000# trailer, and driving empty it feels like you are dragging an anchor. It also doesn't downshift in time when it gets in a pull and this lets the motor lug to far down and even when it does downshift it can't recover. I have talked to a few dealerships and all they say is it is a heavy truck it isn't supposed to feel like it has power. I had a 2000 Chevy before this with 175K miles and it would run circles around this one, and have driven others that have lots more power. I am considering trying to put a 6L80E trans in to get some more gears, but haven't had much luck finding out if that is even possible or if it will help. Where can I start looking on the motor to find a problem without spending big bucks to find out the problem is in my head. When I had the custom ECM programed the guy that did it said my Mass Airflow sensor and the throttle body were way out of sync but he corrected that in his programming. His programming did improve alot but I still think I need more. Thanks for your advise.