I'll jump in here and resurrect the topic... We have an '08 Limited with 42,*** miles and the 20" wheels. I just put a new set of Pirelli Scorpion ATRs on it (came stock with STRs). With the old STRs I was hitting 20 - 21 on the highway a lot. With the ATRs, I lost about 2 mpg.
It also has a LOT to do with where / how you drive. My wife is the primary driver, and takes it to the grocery store, school, dance class (for my daughers) Costco, etc. She doesn't drive like Dale Jr. but does tend to keep her foot in it right up until it's time to brake. She also doesn't let many people get off the line faster than she does. The last tank averaged 9.2 mpg.
When I drive it around town, I'm more careful to get all the coast I can coming up to lights, and can coax about 12 or 13 mpg out of it.
On the highway, with the new tires I can typically get 17 heading West (into the wind) or 18.5 - 19 heading East.