Quote:
Originally Posted by Vman455
I'll rephrase my question: is there a point at which the inertia of a more massive body overcomes the increased rolling resistance of that body such that it will roll downhill faster than an aerodynamically-identical, less massive body? I see people all the time claiming that a heavy car will roll downhill faster than a lighter, but I've never seen any evidence to suggest this is so.
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Good question, and I'm sure there is a scenario where increased rolling resistance would outweigh the benefits of having a more dense vehicle. It just isn't likely common.
I know a bicycle has an extremely low rolling resistance, but I can't coast downhill at 55mph, or coast very far up a hill. A car does much better in this case because of the more efficient aero shape and increased density. This, despite having substantially more rolling resistance.
As far as the advise to keep a car in gear on a downgrade... it's the always safe suggestion and sometimes more fuel efficient. No damage will occur to the car by leaving it in gear, and it certainly will save gas on grades steep enough that brakes would need to be used to maintain proper speed.
All that said, I very rarely encounter a grade so steep that coasting will exceed the safe speed.