I think it all has to do with throttle opening and engine vacuum. Anything that increases torque can potentially help mileage at highway speeds.
I've also found slower speeds are not the answer. You get exactly ZERO MPG when stationary. An engine uses so many pounds of fuel per hour at a given load. The faster you can go for that same load the more miles you travel for that amount of fuel.
For what it's worth i've gotten 20mpg from a 390 4 speed (always have a stick if you want mileage) 67 Ranchero at 4500 rpm with 4.11 gears.(Imperial gallons here) That's with a decently big cam and at that rpm hills don't even move the vacuum guage needle off 17.
