Well, I can tell you that my '11 Hyundai Genesis V-8 gets 30.1 MPG at 75 MPH on the flat, at 4500 ft above sea level, in cruise control. Mileage drops to around 25MPG at 80 MPH, and around 28 MPG at 70 MPH.

Although it sounds like my sweet spot would be at 7... Login to read more.