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Just bought a new to me 2012 g37xS sedan with 19k on the clock. Have driven about 1200 miles and am averaging 23.7. 80% highway, only really spirited on on ramps.
I reset the nav/computer mileage device when I bought the car in July 2013. I have never reset it since.
It shows 25.1 mpg overall in 15k miles.
I do reset the dash mileage computer at each fillup.
My car gets around 23 in town and 27 on the highway and it doesn't matter what octane fuel I use WRT MPG.
I use cruise on the highway, and typically shift at 3k or less, and coast whenever approaching red lights or on long downhills; around here there are many.
YMMV