Congrats! I'm also in a 2017 GT performance package, manual. I'm in the suburbs on the opposite coast, with about 80% city 20% highway, and I get between 18.1-18.5 mpg which I don't think is too bad considering I hammer it around on the weekends. Took it on a long road trip though and got about 24 mpg on the trip, which I was happy with. I think some of the standard mods (CAI and exhaust) helped me with city mileage though. Enjoy!