There’s been quite a bit in the news of late about the nationwide declines in driving and gasoline consumption. Take, for example, last week’s PIRG/Frontier Group report on what those declines mean for the nation’s transportation finances. The report generated some interesting press coverage.

And there’s also been quite a lot of attention to ethanol—particularly the fact that US ethanol consumption has grown so quickly that refiners are starting to bump against the so-called “blend wall,” the point at which no more ethanol can be added to highway fuel without running into legal and/or technical troubles. (For more on the blend wall, see this Congressional Research Service report.)

But the two issues—declining gas consumption, increasing ethanol consumption—actually interact in interesting ways. Ethanol has about one-third less energy per gallon than gasoline does. Combining the slight declines in the volume of gasoline sold with steady increases in the amount of ethanol in the nation’s gas supply, the energy content of gasoline used on the nation’s roads has shrunk at a surprising clip. Take a look:

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  • gasoline - volume vs. energy

    As the chart shows, US gasoline consumption peaked in 2004. Measured by volume, gas consumption has fallen modestly since then. But as ethanol in the gasoline supply grew, the energy in the average gallon declined—leading to a steep, 6 and 7 percent decline in the annual energy content of fuels consumed. All told, the energy consumed by cars and trucks has declined almost as steeply as it rose during the latter stages of the driving boom.

    The upshot: we’re driving less; we’re driving more efficient vehicles; and we’re using less energy-rich fuels. It’s quite a change! And yet it’s been slow enough that it’s gone virtually unnoticed; and it’s certainly a change that hasn’t yet filtered into how we make decisions about our transportation future.