Yeah with me i get that too but i have to keep it a steady 80mph because of the height of my tires. So far my best has been 21 with them. If i start to dip & dive i get at best 19.55 is sooooooooooo boring. I prefer 80. I still get above 20 MPG on the interstate.
x3. There are lots of mods that may or may not help your mileage, and even if they do they still might not pay for themselves in fuel savings over the time you own the truck. Even things like taller wheels, which affect your final drive ratio slightly, can end up weighing more and giving your truck a taller front cross section of air that it has to move out of the way.Bottom line: I say x2 to the "drive slower", that is the biggest thing you can do to increase your MPG.