2003 7.3 E350 box truck with a 14' box and 4.10 DRW. Last 600 mile round trip I got just about 12.0 mpg driving 65 - 70 and going from sea level to 5,000' and back again. I've put about 4,000 miles on this rig since I bought it a little over a year ago and it looks like my average has been just over 10 mpg. My best mileage was 13.4 bringing her home when I bought her 450 mile away and running empty other than my 700lb. Fat Bob in the back. All of my trips, excluding the last one where I maintained 65 - 70, were running the speed of traffic which was probably closer to 75 - 80. My projected frontal area is basically 8' x 10' so imagine pushing two sheets of plywood perpendicular to the wind at 75 mph. It would be interesting seeing what kind of mileage I could get at a steady 55 but thats not likely to happen as long as I'm on the interstates. Loaded, which is the only way the truck gets driven, I'm at about 6 tons. There's no free lunch. Otoh, the van serves as our camper/toyhauler, has plenty of headroom to stand and walk around, eliminates dragging a trailer around, and we can comfortably sleep inside while fully loaded with two road bikes and all of our gear. Modifications follow every trip and it just keeps getting better.