contrary to the pictures, i found a CFD pic of an openwheeled indy car and from my understanding vortices are desirable. Wouldnt this mean that the low pressure zone behind the truck is filling with air from the vortex thus reducing drag created by the vacuum?
edit, link to pic since pics wont display properly (could be my browser though i suppose)
http://aprperformance.com/index.php?...=172&Itemid=44