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Originally Posted by mtch
A note on drag coefficient differential between top up and down. I went cruising top up a couple months ago and pulled from 70-148 using 4th and 5th gears. The car pulled hard and felt like it had plenty left when I let off. A couple of weeks ago, I tried the same pull and could not get above about 128 top down. I pushed to redline and could not climb any more. Not only would it not climb as high, it took much longer to get there.
At one time, I knew the equations for relationships between hp, vehicle speed and drag coefficient. I don't know anymore, and I'm to lazy to look them up, but experiencing the difference was astonishing to me
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It's not a really simple formula. It also has to include the rolling resistance of the tires, which varies from tire to tire and you'd have to know what the wind was doing.