I get that kind of milage.
Mines a 06 FX4, auto tranny, stock tires, AC on all the time, and 4.10 gears. (mods on my web page)
Over the last two months I've been back and forth between Tenn and Mi about 4 times as well as local driving. Driving *easy* but yet running 70-72mph on the interstate, and 45-55 local, I've seen as low as 20.0 and a high of 21.75
I have noticed that 65mph vs 70mph make a significant difference too. (scanguage instant mpg readings)