I've just been thinking about the effect of altitude on coast down tests.
Consider that the gravitational potential energy = mgh, therefore with only 1 metre of elevation (not noticeable when evaluating an area for altitude), the GPE = 9.8 m, where m is total mass of car. (All units of energy are Joules)
The kinetic energy at 100kph is E = .5mv^2 = 385m. The KE at 90kph is 312m. So the difference is 53m. Just a 1 metre difference in altitude between start and finish is enough to throw your results off by 18.5%.
That's not even counting the effect that headwinds or tailwinds will have on your results, which will be significant. Maybe the terminal velocity on a known hill would be a better test? At least that way you take the altitude out of the equation, athough you will of course need a dumpy level to calculate your CdA.
I'm coming to the conclusion that to measure automobile CdA properly, the minimum tools required are at least a dumpy level or highly accurate altimeter.
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"Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed."  Isaac Newton
