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Actually this air deflector does help your aerodynamics. They have been putting similar deflectors on the back of school buses to try and improve aerodynamics. I read an SAE paper where they said they could reduce the drag of a bus by about 14% if I remember correctly.
It keeps a boundary layer attached to the back of the vehicle preventing it from becoming a turbulent vacuum. You do get a turbulent vacuum behind the back of the deflector however the reduction of the vacuum behind the majority of the back of the vehicle more than makes up for it.
And as noted it helps keep your rear window clean too
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