I got a 2012. Purchased it new about 4 months ago.
The first 4,000 miles, I can say that average mpg for me is about 15-16 combined. City driving is terrible, if that's all you do. Highway driving is good.... about 18 to 19.9. I say 19.9 because I had never been able (with uncovered bed) to eclipse 20mpg with any amount of sustained driving although I got awfully close.
About 3 weeks ago, I bought a Leer cab height shell. I don't think it has done anything for the city mileage. Added about 200# to the truck although the 3/4 tons don't really seem to notice. BEFORE buying the shell, I researched through all the forums and many folks were saying that mpg would drop with the cap, even on the highway due to extra wind drag. I was expecting for the worse but needed the cap because I was making a long trip to haul lots of paper files and needed the cap. So 2 weeks ago, I drove 4,000 miles round trip with the "new" cap. Results were pleasantly suprising: On the empty leg to Missouri (from California), For the most part, I averaged about 21.5mpg.... up hills, down hills, with traffic, with headwinds.... Best tankful I got was 23.7mpg. Worse tank I got was 18.7mpg. On the return trip, I loaded up about 2500#'s in the bed. Another 5,000#'s or so in a 6x12 tandem uhaul trailer. This resulted in a worse mpg of 11 through ferocious kansas winds. Best I got was 15.7mpg once the winds were somewhat calm. This was done with truck in tow/haul mode (as members here have suggested). Seems to tow fine in "normal" mode too but as the truck was new, I wanted to err on the side of caution. I do believe that mpg's take a hit when in tow/haul mode although by how much, I do not know. Transmission temps during the loaded return leg varied from 140f to 165f depending upon how much wind, whether on hilly roads or flat. Deccelerating downhill is likely to result in as high tranny temps as pulling uphill. Flats is when the tranny temp is lowest, although I have heard that anything less than 200f is fine. Anyhow, back home safe and sound now, and the truck now has 8,000+ miles on it.
I should say that the transmission makes this truck such a joy to use. It shifts smooth as silk and is a perfect match for the engine. I am also expecting the truck brakes to last probably forever. That's because the combined automatic engine braking and high compression of the motor means the truck slows down gracefully whenever the gas pedal is let up on. While this does take some getting used to when migrating from a gas motor, once you do, you'll probably like it a lot. Not nearly as extreme but think hydrostatic tranny on a tractor and you'll get the idea. My wife's Chev Tahoe, by comparision, on downhill treks feels like a runaway bowling ball and toasts the brakes rather readily if doing lots of hilly driving.
