ive been wondering about this to, but apparently they are only advertised as weather deflectors. and never for any aerodynamic gain, wich they certainly would do if there was any.
but i noticed that all modern trucks have these wind guides to the sides of their cabs, to help keep the airstream attached.
why not place such guides on all the edges of our cars where the airsteam detaches? like above the nose, above the windshield, around the A-beams... would that do any good?