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.
On our last trip we got 16.7L/100km (~14mpg). That's weighing in at 3,530kg with the bubble roof. In this neck of the woods every trip starts off with a gnarly headwind through the foothills and numerous mountain passes. This particular trip also included a "quick" 20km jaunt up a 800m fire road so I was pretty happy with that mileage and it's close to the best we've ever gotten. Normally things are closer to 18L/100km. For comparison, our Crosstrek on day trips gets at best 8L/100km (~30mpg) and I'm a speed limit kind of guy.
Some of you must have some seriously flat roads to get the mileage you do!
Oddly enough, my best logged mpg was on a narrow, winding, mountain highway in a snow storm. It was about 100 miles and went from sea level to the summit at 1156 ft and down to sea level with a few ups and downs along the way.
The Turbo was on hiatus, speeds were steady but slower, no quick starts or stops. I by the fuel used I saw 11 L/100 km (21.38 mpg). From a rig with a big box weighing 9600 lbs. Its corners on the top have a 6” radius... and it’s frontal surface is 88” with and it’s about 102” tall. Unlike a standard box truck, it spent time in a wind tunnel during its design phase.
The moral... the more you push the pedal... the more it’ll drink.
The best I’ve gotten on a complete tankful is 35.5 MPG in adiesel Grand Cherokee — all highway driving, at reasonable speeds and little wind (tail or head). I went 835 miles on that tank. Wish my bladder could last that long.
2004 Ford V10 E350 eb SMB. Quigly 4wd and fixed high roof. On the 1100 mile trip home to Oklahoma from Virginia, I kept the speed averaging 65 mph. Worst tank 12.7mpg. Best 13.8 mpg. Water and fuel tanks were full, but we hadn't loaded any other gear yet. We just had new bumpers built, and have gone to larger tires, so I expect those figures to drop.