Lighter weight, lower ride height (aerodynamics), higher tire air pressure (reduced drag) and synthetic fluids (reduced friction) are the best ways I know to increase milage. Like other have said though, the good tuners do increase milage by a small amount.
Usually the only real way to increase mileage in a vehicle without changing parts is by increasing timing. A lot of the boxes/programmers do this. IMHO viewed purely from economics the pay back time would be excessively long. You'd do better running skinny tall tires or something. Personally I'm very pleased with the mileage the truck delivers as is. I don't know of any real way to substantually increase mileage without spending a lot of money compared to the potential return or degrading the utility of the truck.
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