Tracked 669 miles through Fuelly so far on my 2015 GSW (plus the current tank about 260 miles). Pretty happy with the mileage.
First tank was 36.6 with a fair bit of in town and traffic driving (quite literally I believe the first full tank ran through the car since it was fixed with Phase 1). Second tank was 37.8 mpg with slightly more highway mixed in. Keep in mind about 8 miles of my commute home from work is about a 6% incline highway at 65 mph or so on a semi windy road! Also, living in a rather hilly part of California.
I am very happy with this car so far, the mileage is more than double what I was getting before and it rides really smooth with a good sounding stereo! I will be topping off tomorrow before I head to school. Always laugh at the pump how quickly it pumps 7-9 gallons compared to the 20 gallons on average I was pumping before, not to mention the price difference!