Until 2007, the way the ratings were calculated was tremendously outdated and didn't in any way reflect how people actually drive. The EPA's original test say "highway speed" is 60 miles per hour, for example, and the vehicles were run without the heater or A/C on. A "high-speed" test was added for Model Year 2008 vehicles, along with an A/C and cold weather segment, but the A/C is still off in four out of the five segments.
Still, the test is not yet perfect, by any means. It still short-changes start-stop technology, for example, and diesels do not seem to get a fair shake. The test also requires a full minute to get up to 50 mph. The result, especially for CAFE standard calculations, is that the old testing methods – some set way back in 1978! – have created a lot of vehicles that can pass the test just fine but could be a lot cleaner if the government recognized the various benefits (something Mazda has certainly taken to heart). The real problem here is that, thanks to these outdated testing methods and ways that automakers can get credits for introducing technologies, the much-touted 54.5 mpg CAFE standard is actually going to result in real-world efficiency numbers of around 40 mpg.