Thought I would put my .02 in here. I have gotten as high as 24.5mpg and see 20 mpg regular. My best was achieved cruising at 55mph across Norther AZ before the speed limit there was raised to 65. Aerodynaic drag is VERY important. If you add extra's to you rig like bug sheilds running boards, visors, bigger mirrors they will add drag and increase the need for power to over come the added drag, more power means more fuel used. As you increase speed aerodynamic drag increase by the square which increases need power by the square. In other words if double you speed from say 50 mph and it takes say 40 hp to travel at that speed and you double the speed you are traveling at to 100 mph it will take 4 times as much power or 160 hp not double. This is why bumping your speed up increases your fuel useage so much. So if if your in a mode trying to get max fuel ecconomy then slowing down from 80 mph to 65 will make a difference. This is also why GM spneds so much time and money in wind tunnels refining designs to reduce drag. Auto makers have no also designed new Pickup trucks so that the tailgate produces no extra drag so taking the tailgate of or down will not produce better mileage.