I have a new 2013 actually. The best gas mileage I have gotten so far is 16.3. I am averaging 15.3 now after only 5 fill ups. This is the 3.5 liter twin-turbo engine of course. I thought and hope I will get better, but so far it is a lot of fun to drive.
I am considering installing a AFE Stage 2 cold air system on it. I also heard the 4" Heartthrob exhaust system allows the engine to breath a lot more and increases performance and gas mileage too. I am not sure if I buy into all that though, and I am leery to proceed.