Pardon me while I expand this topic a tiny bit. There's some really awesome aero packages for rally cars -
LINK - which may or may not help in the mpg dept. I thought this link kind of applies since we are talking about racing wings being re-purposed for mpg. There's ready made bolt on spats, spoilers, wings, vortex generators, etc. Which seem like they would be higher quality finished products than our creatively used lawn edging. Well at a higher price of course....