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CD is a number based on the ease of a shape to move the air aside as it passes through it compared to a billboard of the same frontal area. A better term is CDa, which takes into account how large the shape is seen from the front.Given two vehicles that have identical CD numbers, the larger vehicle (higher frontal profile) will have to move more air aside, and will have a larger CDa, thus lower miles per gallon. Other factors influence, such as rolling resistance of tires, friction of powertrains, and efficiency of engines.
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