I just towed a 6' x 12' enclosed box trailer over several hundred miles. I weighed the setup on some truck scales at a truckstop and everything weighed 12540 pounds. (My truck normally weighs right at 7000 pounds with a full fuel tank, me, and my honey. ) My gas mileage was 18. 22 MPG. This was traveling on relatively flat highway at between 55 and 60 mph. Pretty amazing huh? I was also chatting at one of my fuel stops with a guy that had an '03 Cummins. He was pulling a 6000 pound travel trailer at the time. I asked him what kind of fuel mileage he got (he had an automatic) and he boasted 14. 2 with the trailer and between 17. 1 and 17. 3 unloaded. Hmmmm... . I'm glad he didn't ask me what my fuel mileage was. In all fairness, he did pass me on the highway a short while later at about 65 so maybe he would get a little better mileage if he was in the slow lane with me. 
I also met a guy who had a maxipad and he was happy to be getting 17 unloaded also. His was an automatic also. He was a nice guy and I let him borrow my road atlas but he was just another lost diesel soul. I love my Cummins!

I also met a guy who had a maxipad and he was happy to be getting 17 unloaded also. His was an automatic also. He was a nice guy and I let him borrow my road atlas but he was just another lost diesel soul. I love my Cummins!