Live in the hills of NW Arkansas at 1200 ft.
2008 Dodge Cummins diesel 6.7L - 350 HP - 6 speed Auto
Empty back and forth to work: 15.6
Empty on the highway - 19 to 22 (according to direction and velocity of wind)
Loaded at 11,000 lbs.: City 15 to 16
Loaded at highway speed 12.5 to 14 (according to direction and velocity of wind)
July 23, 2012 Fayetteville, AR to Broken Bow, OK and back to Fayetteville, AR - 396 miles - I averaged 22 mpg running empty 65 & 75 mph - no wind - and I still have almost 1/2 of the tank of fuel I filled up in Poteau, OK on the way down. Amazed me the fuel mileage because this is the absolute best the truck has ever done. I have averaged 19.8 according to the overhead several times going to Dallas, TX and back.
I have hand calculated many times and the overhead is close, but always under by .2 or .3 mpg.
Truck has 72,700 miles on it and 2/3 of those were loaded.
Pictures are on I-80 Rawlins, WY October, 2011 with a nice tailwind - I never reset the overhead display from the time we left Arkansas to Washington State via Missouri, Iowa, South Dakota, Montana, down to Oregon and back home through Utah, Wyoming, Colorado, Kansas, and Oklahoma. My fuel gauge, overhead MPG, and speedometer are the most accurate I have ever had.