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Originally Posted by denverpete
It's difficult to say what kind of penalty you'd have with the top down. That would be dependant upon the car's drag coefficient up versus down. But, my experience is that the top down significantly alters the aerodynamics of the car - and not in a good way. So, I would also expect to see a corresponding decrease in fuel efficiency.
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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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