I would have to assume that the best gas mileage would be achieved at the speed when the car shifts into fifth. For my car, on a level road, that seems to be about 45 MPH.
I would guess that 45-50 MPH on a level road over a long distance would yield some pretty awesome MPG. Unfortunately, it's hard to find that kind of situation.
On a related topic...I have noticed on long highway trips that the engine labors MUCH less below 65 MPH than above 65 MPH. I've made it a habit to set my cruise at about 64 MPH. Otherwise, even small hills cause the transmission to downshift. It's relaxing to ride in the right lane at 64 MPH anyway!