Purely by Accident
I've got 58 mpg average (indicated) over a distance of 13.6 miles at an average speed of 50mph because of a fault in my servo link to the adjustable spoiler. The link was disconnected and it was just hanging by its shaft.
Either:-
a) The vertical free flap caused more angled deflection of the air to stick to the hatch surface (which angles at 35deg from horizontal) or
b) The wind force is weighted against the 150g flap so that it opens more as speed increases or
c) both
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