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I'm having doubts about the 70-50 coastdown. The problem, as I see it, is that there are multiple forces acting on the car, other than aero drag. I've played around with the math a bit, but I don't see any way to resolve the portion attributable to aero drag. So, it is not possible to determine from the coastdown how much the aero drag is changed. It is true that the "other" factors which I can identify, primarily tire drag and drivetrain drag are constants, but there doesn't seem any way to decide on the fraction, and therefore no way to get very close to an improvement factor for aero drag.
I suppose one could resort to an approximate fraction, and that would result in a derived aero drag change.
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