http://ecomodder.com/forum/showthrea...nnel-1924.html is an excellent thread on this. Bearleener found a handy rule where reducing your coefficient of drag by .01 is equivalent to shedding 220 pounds. However, I believe that figure is only accurate at a constant 60 mph. Since weight has a greater relative impact at lower speeds, and especially under acceleration, the practical equivalent should be considerably less than 220 pounds.
Here
Measure the drag coefficient of your car - Instructables - DIY, How To, ride is an excellent instructable that should be able to tell you how much of an improvement those wheels make. Of course, the simplest solution is always to hook up the ol' scangauge or neighborhood friendly gas station and take em out for some test runs...