I was going to add that you should look to mdcc_tester for the informed science. Thanks for that.
I was thinking more about the air going between the front tyre and fork crown, I reckon a larger gap with the right shape would have a lower drag coefficient than a 1 to 4 mm gap with a blunt profile, as air doesn't like being forced into little gaps and tends to bounce about.
so did LOOK
http://cdn.mos.bikeradar.com/images/news/2008/08/01/Look_2009_596_crown_clearance-850-65.jpg