I often pull a trailer weighing around 10k lbs. Under those conditions, I typically see 10mpg while climbing from sea level to the Mojave Desert (around 2800 ft). 16 mpg on the way home. 13 mpg on flat freeway.

Without the trailer, I see about 16 combined, and about 21 on the highway. But I usually drive past 70 mph. Fuel mileage varies dramatically with rpm on my truck. I would imagine it does with all of them. I have 3.73:1 axles, and it's a 4X4 with oversized tires. I'm sure that has a little something to do with it. Plus, I don't often specifically drive for fuel mileage.