I have an 04 GMC extended with the 4.8. It has 32k on it and I'm getting absolutely horrible mileage. I average about 9.5mpg. Its all city driving, but I should be getting at least 14 at the lowest. I had a 96 3/4 ton w/ the 5.7, almost 200k and I was getting a regular 11 mpg and i drove it a lot harder than i do this truck.
I thought it might be the fuel filter, but its in the gas tank on the 04+ models and not supposed to need to be changed. Other than the air filter, I dont know what else it could be. Even then, I dont think an air filter would be so constrictive that it would hurt my gas mileage that bad.
Is this something I could take to GM under warranty??
Truck seems to be running great. Great pick up, I can chirp the tires at 30mph. Its been serviced by GM before for the power steering steering. Im not sure if they do a check on everything or not though. Ive hooked up my code reader to it and nothing came up. I changed the oil recently...Im lost...any suggestions?