I assume you mean a tapered fairing behind the wheel.
I made one out of cardboard once
I didn't test it independently to try to gauge any fuel economy impact, but I did tuft test it to ensure there was attached flow (there was).
From:
http://ecomodder.com/forum/showthrea...tml#post143382
Of course, you'll be sacrificing approach/exit clearance angles if you mess around with this kind of thing. Have to watch where you're driving or it'll be left on the road somewhere.