OP, I'm not getting as good a gas mileage as I expected either, but I've heard that gas mileage is a bit poorer on a car when it is brand new, until it gets a few thousand miles on it. I don't know much about this--maybe someone else here can clarify, and also tell if that is just rumor, or if it is something that applies/doesn't apply to the goat.
General info, for mileage comparison: Previously, I had an '01 Z28 Camaro, auto, that I got 19.5-20 on in daily driving, driving conservatively most the time but sometimes stepping on it more to pass, etc. This would be going to work, groceries, etc.
I drive my '06 GTO (M6) the same way and, from what I saw posted here, expected similar mileage. Unfortunately I'm now down to high 16's low 17's. I didn't buy the GTO for fuel economy (of course) but am getting lower than I expected. Have high-500 miles (580?) on it now. Was rusty with a stick shift as it had been many years since I'd driven one when I got it, but now got the main parts down, though I have room to improve in smoothness and quickness of shifting. Drive generally conservatively, especially as I want to be easy on the car the first 1000 miles or so (I've stepped on it harder 2x due to necessary situations). I live about 6 miles from work. Driving is "mixed"--mostly on roads that are 35-45mph and have lights every so often, but often you get stuck behind slow traffic, or someone slowing to turn, etc. Take the "highway" to work but it is usually packed with stop-and-go traffic. I guess all told my driving ends up being weighted toward city mileage. Maybe 60-65% city remainder highway?
By the way, what are the roads that are 35-45mph with occasional lights (not every block but intermittently, maybe every 3 blocks on average) with reasonable traffic considered? City or highway?