I'm starting to think that you need to get 25k+ miles on a Duramax to get consistantly good fuel economy (my '04 LB7 was the same). I was stuck in that 18.5 to 19.2 range for a long time (this is on long Interstate drives), but this past week, I've been getting 20+ mpg with ~26k on my LBZ. Monday I got 20.42 mpg & today I got 20.07, both 300+ mile trips at 70+ mph, hand calculated & using my GPS to track total miles.

Not that I wasn't impressed before, but I'm REALLY starting to like this truck.