Had an 03 Silverado 1500 Extracab with 5.3L and 3.73 gears and it averaged about 17mpg highway with a lifetime average of 14.6 and that's towing sleds in winter and camper in the summer. It would smoke my newer truck both gas mileage and power. On trips I could squeak 18-20mpg if I stayed under 70mph. Picked up a new 09 GMC 1500 SLT Extracab that also has a 5.3L with 3.73 gears and it's a dog both performance and gas mileage wise. Old truck could smoke the tires in 1st/low just by stomping on the gas. I could also clear a 2mi 7% grade hill at 100mph if I wanted to. New truck couldn't spin a tire if I wanted to, gets about 13-15mpg on the highway and can only do 86mph up that same hill. The 09 has 18" wheels where my old truck had 16.5". Even cruising 60mph on the interstate I can only get up to about 17mph and if I drive my usual 72-73 I get about 12.8-13.2 mpg. I talked to one performance shop and they said it's likely a different, (less efficient transmission) and the drive by wire. I could see the drive by wire taking the WOT away at the start but it should do WFO once rolling I'd assume. Mentioned it to dealer and they said unless it triggers a code there is nothing they can do. My buddy has an 08 Crewcab and we set the cruise at 70mph on this long flat, his was getting about 23mph and was in 4cyl mode, my truck was in V8 mode and getting about 15. When we race he blows me away with any side by side comparison and I assume his truck is heavier being a crew cab. He also has 5.3L with 3.73 gears. Buddy has a new truck with the 6.2 and got 18.9 on a long trip walks all over my truck both in power and gas mileage. Is there anything I should do to or is it the nature of the beast and there are just good ones and bad ones.