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I have done coastdown tests on my Scion xA, using that Instructables page as a guide. The spreadsheet shows the *critical* nature of the RR coefficient. A small change in the RR number yields a big change in the Cd. Since you are only estimating the RR number, the Cd is relative. You would have to have done ABA testing on the same day, in order to "know" what the Cd changes are.
That's a lot of testing, and it is still an estimate/approximation and it is relative to your vehicle only.
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