Zip-tied coroplast wheel disks.
http://ecomodder.com/forum/showthrea...html#post37306
MetroMPG's method from 2008 for attaching fender skirts. These are nibbling at the edge of the problem; the things that would make a sizable difference would be a comprehensive attack on the underside (diverter/belly pan/diffuser and wheel spats) [and/or] a sizable boat tail mounted on the trailer hitch you'd have to add.