I've got a 2003 3500 chassis cab with a flatbed. It has 4.11 rearend and Allison. I'm getting an average of 5.5 mpg with a low of 5.3 unloaded last week. I've never gotten better than 6.0 mpg. Chevrolet is saying nothing is wrong, which I believe is nuts. The computer "tells" them there is nothing wrong. I think the service guys believe in their hearts that something is not right, but Chevrolet wont authorize them to dig deeper without the computer saying something is wrong. I'm frustrated with the situation and wondered if anyone else is experiencing this and possibly has insight into this issue. The guys around here are getting 9-12 with highs around 14 and the worst I've heard of is 7. The 7 was towing in the winter with lots of warm up time after starting.