i haven't fully read this site but it seems to raise some interesting points.
an interesting point raised herein is that when considdering rear aerodynammics the edge of the roof is often considdered but parts of this article point out that the sides could be even more important.
although the site also deals with retractable tail cones and such, and proposes an overall redesign of car aerodynammics, most of these could be applied to conventional vehicles to a certain degree.
also interestingly, some information is given on possible benefits to a tailfin.
although i have to look into it more in detail there's a lot of inforation here wich i havent seen anywhere else so far. inspirational if nothing else!