From: Wired
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But there’s a problem: Even if the car companies do, they don’t. Or at least, no one has any way to know if they do. Because the EPA’s test to make sure automakers are hitting their CAFE numbers—the sole federal, legal requirement that cars get more efficient—probably doesn’t work. At all.
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Meanwhile, the EPA doesn’t know exactly how its CAFE testing correlates with actual results, because it has never done a comprehensive study of real-world fuel economy. Nor does anyone else. The best available data comes from consumers who report it to the DOT—hardly a scientific sampling.