this kind of thing:

http://www.nosecone.com/aetruck.htm

you might also consider improving the rear end of a blunt trailer too:

http://www.airtab.com/en/

http://www.buyairtab.com/

more scientific info on the general idea:

http://people.clarkson.edu/~visser/research/drag/truck/

don't forget the tow truck

http://tinyurl.com/4ck68d

http://autospeed.com/cms/A_108656/article.html

good luck