Drop your speed. Reduce your weight. Pray for tail winds.
I drive at 55 with auto cruise on and try to stay in right lane. I get approx 9.5-10 MPG on flat terrain. Speed is set using my garmin to compare with chassis speedometer.
What is involved in installing headers on my class C E450 chassis large block ford engine I'm just investagating the possibility. The rig is now payed for in full and I'm looking at my options for better gas milage to help the overall operating costs of fuel usage.