Post your MPG in Diesels.

2005 Dodge dually quad cab 4x4 6 speed manual 373 gears.
loaded 12,400 lbs hwy 14mpg, city 12 mpg
empty 18 mpg hwy, 15 mpg city

I have an 02 F250 with 7.3 and 373 rear end. Running empty, at 55mph i get 20 mpg, normal driving running 72 or so I get mid 17s. With fiver in signature at 65 I get between 10 and 11. Truck has 210,000 miles and has had minimum repairs.

With truck in my sig I'm getting 14-15 around town, and 10.5-12 towing 11.5I. These figures are mostly on the flats. Don't do a lot of hwy miles unless I'm towing.

Only figure my milage by how much fuel is used during a trip.
Last trip of 6456 miles used 475 gallons of diesel.
That's 13.6 miles per gallon.
Two weeks of that trip the camper was unloaded so the truck was running around empty.
The fuel cost me $1864.66.
So the average gallon cost $3.93.
Trip was May and June this year.

My truck
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My truck is the one in my signature an '06 Ford SC longbed automatic 3.73 with the zoodad mod, walker big truck muffler, intake elbow, and I run PowerService Diesel Kleen in every tank.

You didn't describe the MPH which makes a lot of difference. In any case, I've got close to 20 at slower speeds of 55-60 unloaded and 14.5 loaded at the same speeds with some hills.

My more typical mileage at 65-75 are 18.5-19 and 12-13 with a combination of hills and mountains. I don't really have a clear city mileage since I tend to not drive my truck very far in city.

I can't drive very far without dealing with significant hills and mountains considering I live between two mountain ranges in Western WA.

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.