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 Since the amount of carbon dioxide released from fossil fuel combustion is directly related to the amount of fuel burned, fuel efficiency is an important determinant of overall emissions of greenhouse gases. Today's cars and trucks burn fuel 35 percent more efficiently and with 95 percent less emissions of pollutants – excluding carbon dioxide – than 30 years ago, but the continuing increase in vehicle miles traveled has slowed our progress towards environmentally sustainable transportation.
In fact, carbon dioxide emissions from the transportation sector increased approximately 10 percent between 1990 and 1997. This growth is a result of two trends: 1) average fuel economy of the new personal vehicle fleet has declined, and 2) the number of miles driven by Americans continues to rise.
The drop in fuel economy is largely a result of a shift toward larger vehicles, such as sport utility vehicles and minivans that have lower gas mileage than cars. While some vehicle models may be improving mileage efficiency over time, as a nation we are still buying less efficient models. The causes of the increase of driving include declining crude oil prices, population shifts to urban fringes, and other demographic factors.
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