Supercars aren't expected to get excellent fuel economy--and most of them live down to that promise, with single-digit city ratings.
The 2012 Nissan GT-R's at least made an effort to improve its numbers. This year, its EPA numbers are up to 16/23 mpg, from 15/21 mpg. It's still not great, when you consider the GT-R is nominally a four-passenger car, and when today's Corvettes and Porsche 911s manage highway numbers in the high 20s
However, for a supercar, those gas mileage figures are fairly solid. Mostly, it's due to the GT-R's big twin-turbo V-6, which turns out huge horsepower while using less fuel than a comparable V-8 or V-12.