Measure the drag coefficient of your car - Instructables - DIY, How To, ride
Very interesting stuff.... I plan on doing this with the Si. Some interesting comparisons could be:
Stock vs. aftermarket tires (rolling resistance)
Stock underbody vs. DX-G underbody
Windows/sunroof open vs. closed
Spoiler vs. spoilerless
Stock vs. HFP body kit (or aftermarket)
Weight reductions (for rolling resistance)