HotRod magazine did a thing on aerodynamics at a windtunnel location, & the 'expert' that runs the windtunnel says a dirty car has the same drag as a clean car, it makes no difference
it doesn't say it here, this is the article, but the online articles tend to leave out stuff thats in the magazine
http://www.hotrod.com/techarticles/b...ips/index.html