Best thing you can do is look at the terrain and keep the rpm as low as possible w/o bogging in 4th. For example I ended up being relocated for a while into the woods. What this did was put me off the highway at 70-80mph and onto roads that were 40-45mph. I drove 45-55mph and got better mileage than I did at 70+.

If the terrain isn't hills etc..you can do better by going a bit slower.