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I think it is impossible to accurately calculate Cd* by coastdown tests, without very sophisticated data gathering (including real time wind/yaw) and very good modelling (including all rotational inertias).
Given the high speeds you are using, it might be possible to measure some relative changes in drag. Do the tests again with windows up / windows down and see how much consistent difference in speeds you can record after 50 seconds.
(* let alone to three decimal places!!)
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