Best magnum MPG?

My wife's 2001 Durango (5.9-4x4) averaged 13.5 mpg whether it was driven up hill, downhill, pulling our trailer, or not. It was consistent, but not good for gas mileage. My 99 Dakota exented cab (5.2-4x4) has a best of 15 mpg in town. Both vehicles have 3.55 gears in the rear.