I'm impressed with the 26.7mpg on flat ground doing 67mph. Best I can get like that is maybe 21 or 22 with the AFM/DoD, but I have the smaller 5.3 (LY5) w/4psd and 3.73 gears. In order for me to see that kind of mileage on flat ground I'd have to be down around 45mph.
maybe i hit 10th gear which is like super mega overdrive?
not to mention the aero tweaks of the new trucks vs old