Even though I play golf and have at least a basic understanding of fluid dynamics and how that applies to golf balls, airplanes and cars, I was stunned by this finding.  
 
Mythbusters, testing the myth that a car will get better gas mileage dirty than clean (because of a "golf ball effect") found out that a dirty car is less fuel efficient. But...they took the myth a huge step farther and made a clay covered car with golf ball like dimples all over it. The result was an 11% increase in fuel efficiency at highway speeds.  
 
http://dsc.discovery.com/videos/mythbusters-dirty-vs-clean-car/