Very cool. I wonder if it gets the driver any hassle from the law due to the extra length.
NASA did some aerodynamic testing of a very similar mod to use in that exact application for fuel economy. There's a link to a PDF around somewhere, possibly in my meta-sig.
Edit: Found it. I was wrong, it wasn't the NASA link, although they have a partial boattail that looks similar. This is the one I was thinking of:
http://www.greencarcongress.com/2005...at_tail_d.html
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