[newtonsfirstlaw]

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.