[Diesel_Dave]

DirtyTorquer, you're on to the root of this.

Work output of the powertrain necessarily increases with speed because the aero drag is increasing with the square on speed. So for your mpg to get better at higher speed, something about the powertrain operation has to change to allow you to produce more work that you were before with less fuel than you were before. I.e. if the higher speed means you need X% more work output, the powertrain needs to have become >X% more efficient. That scenario is possible, but pretty rare.