Welcome to Ecomodder! I'm sure that with your background, your brain will get picked often.
I don't know of any online software we have access too, but there are a few spreadsheets floating around dealing with aerodynamic calculations and cost/benefit analysis.
Also, there's a site called Flow Illustrator that makes rough aerodynamic flow movies from an image you upload (such as a profile of your car).
http://chernyshenko.sesnet.soton.ac.uk/FIHome.htm
Cheers!