Well seeing as the 7.3L and 6.0L can get upwards of 20mpg in some situations... I'd hope that the 4.4L could get 25-30+ mpg
The new ('08) emissions diesels aren't getting anywhere near that. For a D-Max, 17-18 is about the outer limits. Keep in mind diesel also comes with about a buck premium to regular. Sooo... it's an important question. Silverado will be out with a hybrid and a small diesel in '09- make for an interesting Ford vs. the other guys thread when we have more specs.