Quote:
Originally Posted by Awd180
Drag coefficients that you read on a chart are computed at ~40-45mph, whereas when you are going slower its lower and faster its higher. You can speculate values but without quanitified test results your values aren't critical and therefore useless.
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"Useless" is an overstatement. True, Cd changes with speed but the difference is small enough that for all intents and purposes we can assume it's constant for the speeds at which a road vehicle typically operates. There's a thread around here on this subject with sources cited.