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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  1. Anonymous - April 6, 2008

    Lets look at some facts based on real world data. I personally spent US$350-ish on an upgrade to allow my ’98 Wrangler TJ to run ethanol.
    E-85 is still (**STILL**) at $2.199/gal while regular unleaded gasoline is at $3.29/gal and rising.
    I get 15-ish mpg with 85 octane gasoline, and 13-ish mpg with E-85. Yeah, I top off the tank every morning.
    Cost per mile using gasoline : $0.203
    Cost per per mile using E-85 : $0.169
    Basically, E-85 is $0.034 cheaper per mile!!! Whoopdedoo!!! Right? Big honkin deal!!! Yeah?
    EXCEPT I drive 100 miles each day to work and back. Lets do some real simple math:
    $0.034/mi * 100mi/day = $3.40/day (savings)
    $3.40/day * 20day/month = $68/month (saving).
    $816 saved in a year! That’s not too shabby, in my book. Thats a MUCH greater savings that swapping out a 75watt lightbulb for a low-wattage fluorescent. Thats even a better savings than keeping your thermostat at 68deg.
    With gas prices increasing at uncontrolled rates, I pity the idiot who doesn’t choose American fuels. But hey, it’s your money… mine’s in the bank!

  2. Anonymous - April 6, 2008

    Oh, I actually get better economy if I keep my speed at 55mph or so, but… what can I say? I’m an American. ;)

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