Got a 5x8 trailer today and made a run up and back. The hatch was configured to the same height as the 4x8 trailer and the Gurney flap was active going up. Indicated mileage going up was 19.1. Coming back empty, I deactivated the Gurney flap and got 18.1mpg. The height of the hatch from ground was 68" loaded and 69" unloaded.
Speed was steady at 60mph. Didn't have cruise control as the brake switch wasn't working correctly. Will have to fix that soon...I miss my cruise control.