The consensus here is that VG's are useless for fuel economy, and sometimes detrimental because they sap energy to generate vortexes. You'll want to research boat tails, belly pans and gap fillers. The deflector is only the beginning!
Boat tail:
http://ecomodder.com/forum/showthrea...d-e-18151.html
Gap fillers:
http://ecomodder.com/forum/showthrea...ta-t-7839.html
Design:
http://ecomodder.com/forum/showthrea...ign-18585.html
Smooth belly pan:
http://ecomodder.com/forum/showthrea...n-s-18427.html
Start reading! 
PS- here's the primer course
http://ecomodder.com/forum/showthrea...rt-c-9287.html
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He gave me a dollar. A blood-soaked dollar.
I cannot get the spot out but it's okay; It still works in the store
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