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Mine sloped up from the cab going back. That increases frontal area and coefficient of drag, which is doubly bad for fuel economy. One that slopes down from the cab to tailgate height is probably the best for economy, but not very useful or practical. A flat bed cover improves aerodynamics.
As for gearing, the taller the better. The 300 has loads of torque and gearing will never be too tall for it. The tallest rear end ratio with the tallest and narrowest tires you can get will likely result in the best fuel economy.
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