Mine 04 was rated at 16/21. Best mileage on a road trip going mostly 75mpg I got just over 21mpg. Outside of that I was averaging 16-17mpg with well over 2/3 of it being highway driving. Mind you this includes some freeway congestion and typical L.A. traffic. Lately I've been averaging about 14.5mpg due to less highway driving and more traffic. New GY Fortera tires may also be a factor but I have never isolated them to be the issue.
I do have 4.10LSD, aftermarket intake and exhaust, SCT XCal tune, stock wheels and tire size. And I'm not easy on the gas either.