I got 33.6 with my 2009 GT on a 62 mile trip through an area with some mild (and not so mild) hills to climb. I tried the same route at 67 and got 33.4 which makes me think the sweet spot for mileage in my car might be around 64 or 65. Running the same route at 80 yields 28.6...
With my typical city driving, I'm sitting at 17.5 if I keep my foot out of it and 15.8 if I have some fun.
I have an app to track my mileage since I bought the car. So far, 9,476 miles with 514.99 gallons burned since late December with an average of 18.91 MPG. I drive about 20% on the highway. Suck, as much fun as the G6 has been I really do wish I had bought that TDI.