Consumer reports came out with a very interesting study recently, The Ethanol Myth, which compared the gas mileage of cars running petroleum-based gasoline versus running on E-85, which is a gas (15%) and ethanol (85%) mixture. The interesting finding had nothing to do with whether ethanol would reduce our nation’s dependence on imported oil, whether it would reduce greenhouse gas emissions, forestall global warming, or help deter terrorists.

Instead, it asked a simple question: is ethanol-based fuel as efficient in existing cars as gasoline. The finding: “you’ll get cleaner emissions but poorer fuel economy … if you can find it”

CR was motivated to do this study because:

The Bush administration has been pushing ethanol as a renewable, homegrown alternative to gasoline. Now, the auto industry is abuzz with the promise of its flexible-fuel vehicles (FFVs), which are designed to run on either gasoline or the blend of 85 percent ethanol and 15 percent gasoline called E85.

So they ran a test using a 2007 Chevy Tahoe FFV (designed for ethanol). They found there are two problems with ethanol.

First, it reduces your gas mileage by about 27%, “from an already low 14 mpg overall to 10 mpg.” Not surprising since a gallon of ethanol has about 70% of the energy of a gallon of gasoline. Now if engines were optimized for ethanol, they could be as efficient as gasoline engines are today. But so might gasoline engines if they were truly optimized for efficiency instead of acceleration and other factors.

Adding to this problem is the considerable debate raging now about exactly how many gallons of gasoline it takes to grow, process, and distribute the ethanol (if Wikipedia is good for anything, it’s for putting the debate on center stage: Ethanol Fuel). Critics claim upwards of 1.5-2 gallons of gas are neeed to produce a gallon of ethanol (in the US, which uses corn).

There is a race to find the processing methods that are more energy efficient, but it will be a happy day when it takes less than one gallon of gas to produce one gallon of ethanol (which, remember, is still the equivalent of 0.7 gallons of gas). But this may require the maturation of cellulosic ethanol production (which uses farm residue rather than traditional product).

Second, they found that, because ethanol is produced and sold in the Midwest, it’s not widely available elsewhere. So maybe the reduced mileage won’t affect many of us, since we’re not going to be able to use E-85.

In any case, the argument for ethanol just got a little muddier. It’s a heck of a lot better when you compare it to the emissions of traditional gasoline. But since it’s the cure du jour for our energy woes, it’s worth kicking the tires–if only to see what needs to happen before we can seriously takes our eyes off of more attainable, if less glamorous, solutions like energy efficiency through improved mileage, better public transportation, smarter commuting.