Is yours a standard 5.9 or a high output? I've heard from several other TCD Dodge Ram truck owners say that the 2003 get the best mileage and power of the stock trucks I have a freind who had a 2005 3500 4 WHD dually that towed the same trailer as I do,24 Ft. Pace Shadow and he uses a 12.5 ft overhead camper and he got better milage than me towing just my trailer, our trailer are probbally real close to the same weight with the cars in them. My truck is a 2006 3500 4 WHD four door (his is to) single wheel with the same gears as his so I tend to believe the other drivers on the earleir trucks getting better mileage, try different power settings on your box and see what that does. I also agree on the speed and wind affecting the mileage Slower is better when it comes to burning fuel I've got as good as 14.5 MPG towing the car trailer, I have a 34 Ft fithwheel camping trailer, it is really tall and the best milage I've got with it was right at 12 MPG going 60 MPG or slower on the first trip with it, the speed limit in Oregon is 55 MPH no matter what I've gotten worst mileage later going faster into a head wind coming back from Arkansas this spring, 8.3 based on filling the tanks all the way up and the speedometer reading The overhead console fuel milegage lies don't believe it