I have driven 75 MPH and have gotten 20 MPG doing it, once. One or two trips doesn't prove or disprove anything.

It's a scientific fact that as you go faster, drag from wind takes more power to overcome. That power comes from gasoline.

I'd say that at a certain RPM point for a specific engine given the right conditions that you might get more MPG by going faster, but overall slower is more fuel efficient.

Mike