A friend of mine has purchased a new '04 Silverado 2500 HD Duramax Diesel Crew Cab. He previously owned a '01 Siearra 2500 Duramax Ext. Cab, and traded it in for the larger cab. The problem is the the fuel economy, in the '01 he would get around 23-27 MPG and now his new one is only getting 10-13 MPG and horrible towing mileage of 5-7 MPG. This truck also lacks the grunt the old one did, not a lot of power when passing on the highway. It has been back to the dealer 3 times and they claim all is well, the computer is not showing any problems and the programming is up to date. A check of the engine oil at the first oil change showed fuel in the oil and the dealer still says it's normal. Has anyone else out there had these problem and if so what was the fix? What should he be looking for as far as power and milage and performance out of the '04 model vs. the '01.