Thundersalt:
Yes, it's a Duramax, but it's an '04 LLY option. The LLY was a second-half-of-the-year model, with more HP than the early version, but quite a bit less power than the new (current) models. I was talking with a guy at Maxton this weekend -- he's got an '06 and an '08 Duramax and says he's never ever gotten anything over 20 mpg. But -- we never drive at speeds over the speed limit. This weekend we spent most of 2,400 miles on freeways posted at 65-75 mph -- and went about 63 or less the whole time. When we've gone faster -- as little as 5 mph faster -- the economy drops drastically, like a couple of mpg. And we use the cruise control as much as possible, which probably also helps get good mileage.
Keep your tires properly inflated, and also -- I run Rotella 5W40 synthetic oil, which gave me a one mpg boost instantly when I started using it a couple of years ago.