This chart shows how fuel economy varies with driving speed for midsize cars and three fuel types, as modeled by Argonne National Laboratory's Autonomie model. Midsize conventional gasoline cars achieve their best fuel economy at 55 mph. The fuel economy of midsize conventional diesel cars declines gradually from 45 to 55 mph, then drops quickly thereafter. The midsize hybrid electric vehicle loses efficiency more evenly between 45 and 75 mph.

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