The 2015 Infiniti QX80 is EPA-rated at 14/20 mpg, which is okay for a full-size SUV its size, if fully used. But if you're not putting butts in seats, or loading the QX80 with cargo, it's one very thirsty, inefficient luxury vehicle.
No matter which edition is specified (rear-drive or four-wheel drive), the QX80 has the same ratings. And Infiniti has no plans for hybrid or diesel QX utes here in the U.S.
While those aren't stellar numbers, they are far better than Nissan's own Armada SUV.