[MetroMPG]

It has been estimated that a 10 percent reduction in Cd can lead to a 5 - 6 percent improvement in fuel economy at highway speeds and a 1 - 2 percent improvement at urban speeds. (source: http://72.14.203.104/search?q=cache:uRXQgiGGpEUJ:www.energy.ca.gov/reports/9712TTSR.PDF+%22reduction+in+Cd%22+%22miles+per+ga llon%22&hl=en&ct=clnk&cd=2)

Though: the base Cd isn't mentioned, and highway speeds aren't defined.