Very good point. It takes the same amount of energy to get and keep the truck down the highway regardless of engine displacement. Pick the engine you want. Then, set cruise rpm to accoumplish this task with the best vacuum and least throttle imput to maintain the desired speed in the conditions you drive in most. With carbuteted engins keep out of the power valve our your licked from the beginning. Electronic Fuel Injection is why modern LS1 vettes can cruise 70 at almost idle and get away with it. The combination of EFI and low cruise RPM with close ratio gearing in the higher tranny gears is the key to best mileage.
