All of this is guesswork unless you actually do the math to determine your mileage.

FWIW, we just took a road trip of about 1100 miles round trip in my wife's Acura 3.2CL Type S. It has a 260 HP engine and we cruised at 80 most of the way. Low mileage was 27, high was over 31, average was about 29.5 for the trip. Obviously the gearing is a lot different and it shows...the Acura turns about 2200 RPM at 80 where my X would be turning well over 3000 RPM at that speed. The Acura is a little more aerodynamic too.