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 12-25-2007, 08:25 PM #8 (permalink) newtonsfirstlaw EcoModding Apprentice   Join Date: Dec 2007 Location: Melbourne Posts: 152 The Mighty Mira - '92 Daihatsu Mira 90 day: 61.32 mpg (US) Thanks: 0 Thanked 7 Times in 7 Posts 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. __________________ "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