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How a Suburban gets 30 mpg (at least in CAFE)

Math is hardWhile digging up background for the CAFE post, we ran across the curious case of the Chevy Suburban that is credited with a CAFE fuel economy rating of 28 mpg — quite a bit better than its actual fuel economy of 15 mpg. This mind-bending math comes to us courtesy of a provision in CAFE known as alternative vehicle credits.

The details seem rather logical at first. For flex-fuel vehicles capable of running on either gasoline or ethanol, the CAFE rating represents an average of the two fuel economies. The kicker: the ethanol fuel economy gets goosed by a factor of almost seven in order to reward automakers for producing flex-fuel vehicles. So the Suburban gets credited with an ethanol fuel economy of 84 mpg, rather than a more accurate ethanol rating of 12 mpg.

The overall result is to greatly boost the fuel economy of the Suburban under the CAFE guidelines, while doing little to boost the Suburban’s fuel economy in, you know, real life. Chevy can allocate these credits to reduce their obligations under CAFE by up to 1.2 mpg per car category.

Of course, most drivers aren’t filling their flex-fuel vehicles with ethanol. A very small base of ethanol supply, combined with the expanded light-truck production made possible by CAFE’s alternative vehicle credits, means that the credits have actually resulted in a net increase in usage of dinosaur-based gasoline.

Despite opposition from groups like Environmental Defense, the provisions were extended in February, 2004.

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