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Your Mileage Will Vary

By Roseanne Pereira

Even if you scrutinize those miles-per-gallon stickers slapped on the windows of new cars, you could still drive off the dealer’s lot in a gas-guzzler. According to the San Francisco—based environmental group Bluewater Network, on-the-road fuel economy may be as much as 10 percent lower than the EPA’s published mpg ratings. That’s because the assumptions the agency uses to translate raw laboratory data into realistic fuel-economy estimates are largely based on traffic conditions from the 1970s. Bluewater’s analysis takes into account motoring conditions in this century, among them an increase in urban driving, more aggressive driving due to traffic congestion, and faster highway speeds.

Bluewater’s Russell Long notes that it was disgruntled sport-utility-vehicle owners who first realized that their SUVs "were not performing anywhere close" to EPA ratings. The mismatch between estimates and reality affects more than your own gasoline bill–it undermines efforts to increase fuel economy standards. While the U.S. Department of Transportation reports that 2001 cars and light trucks averaged 24.8 mpg, their actual on-road economy was closer to 20.