You're going to get different results from different trucks, for one thing. All the major automakers want to be able to brag that not only does their truck tow more, have more power, and beat up all their competition's trucks, it gets better gas mileage too. As such, they put a lot of work into making sure the shape generally works well for fuel economy with the liftgate up. I've seen reports from engineers at two of the big 3 claiming their trucks get either the same or slightly better fuel economy with the lift gate up, and one independant test showing it was marginally better with the liftgate down. The one thing that's consistant is that a bed cover will significantly improve aero, and thus fuel economy.