The mpg on these trucks varies so much, it's really weird...
I remember when I had my 2000, and had a few thousand miles on it, just around real break-in time for the engine, I was getting 14-15 mpg on the HIGHWAY!!
This was an ext. cab, 4x4 with the 31' tire option from the factory, the larger rear axle, and the 4.7 engine.
I asked a lot of people and even with the exact same setup, other owners were generally getting 17-18 on the highway.
Of course the dealership said my truck "was within specs"...
I added a K 'n N filter along with a smoother intake system offered by another Dakota owner, and then I started getting what everyone else was getting in terms of mpg.
Now I own a 2002 4x4 Durango, and on the highway if I keep it on cruise at 100km/h I'm seeing anywhere from 18-23 mpg on the overhead display.
Flat ground, no head wind etc. is where I see the higher mpg's.
Of course, with a brisk head wind, the display can read as low as 13 mpg just trying to stay at the same speed... lol