Why does a car rated for 47mpg fall so far short? The Houston Chronicle features an article on just why EPA gas estimates can be so different from real-world drivers' experience at the pump (or in looking at the dashboard display), in particular for hybrid cars. From the article: "A geometric average of the FTP-75 and HFET results (with city driving weighted at 55 percent and highway driving weighted at 45 percent) produces a vehicle's CAFE fuel economy, which is then incorporated into a manufacturer's corporate average. CAFE is measured using these tests to the present day. In fact, this methodology will be 50 years old when it's used to gauge compliance with the forthcoming 54.5-mpg CAFE requirements in 2025. That kind of continuity is admirable in baseball, but not in transportation. These tests are irrelevant to contemporary real-world driving. For example, the maximum acceleration on either test is 3.3 mph per second. At that rate, it takes more than 18 seconds to hit 60 mph. Even in the horsepower-deprived 1970s, most people were driving harder than that. And the 60-mph maximum speed on the highway test does not accord with the 75-mph truth of today's interstate traffic."
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