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I used the method found on instructables once. It is used to find the coefficient of drag (Cd) of a moving vehicle. The Cd of your car can then be fed back into equations to predict gas mileage differences, like the one in the tools at the top of the Ecomodder page. Basically using the speedo on your car and timing the coast down in 5 mph increments, and running the results through the spreadsheet. I have also done this with my phone's GPS. The GPS is faster for data gathering, but slower for data processing. If you have an android phone, the best program I have found for recording speeds is My Speed. There is another that will calculate the grade of your hills, it is called Speed Pro. Both use the built in GPS, so that is your limiter.
Make sure you do bi-directional runs, since runs in one direction can be biased by wind or a slight incline. Also, make sure to do at least 5 sets, 10 is what ASME recommends, but I never have taken that much time. Make sure to start with a warmed up engine AND drive train. It may be nice to record the outdoor temp, conditions (wind, etc), vehicle configuration (any mods), and tire pressure at the time of the runs (when warmed up).
I would also recommend that if you can do 60-30 coast downs, you may want to do a second set of 30-15 (or whatever) to get the rolling resistence better defined. 60-30 does a poor job of capturing Crr.
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