The auto industry is abuzz with the promise of flexible-fuel vehicles (FFVs), which are designed to run either on gasoline or a blend of 85 percent ethanol and 15 percent gasoline called E85.

Our tests of an FFV and our interviews with more than 50 experts on ethanol fuel led us to conclude that – while the mixture does burn cleaner than gas, and with comparable performance – E85 actually hurts fuel economy and may even increase our need for oil.

We put an FFV version of a 2007 Chevrolet Tahoe through tests while running on gasoline – actually, a mix of 90 percent gasoline and 10 percent ethanol called E10 that can be used in virtually all vehicles – and E85.

While there was no significant change in acceleration when running on E85, fuel economy dropped in both highway and city driving. But the Tahoe on E85 did emit significantly fewer pollutants.

Fueling the growth in FFVs are generous fuel-economy credits that manufacturers receive for each FFV built. The government assumes that an FFV will run on E85 half the time, and on gasoline the other half.

But the vast majority of FFVs may never run on E85 at all. Only about 800 stations nationwide sell it to the public, so the ramped-up production is just adding more large, fuel-guzzling vehicles to the roads. And that has increased annual U.S. gasoline consumption by about 1 percent, according to a 2005 study by the nonprofit Union of Concerned Scientists.

Benefits to automakers notwithstanding, there’s currently no financial advantage for consumers to buy an FFV. Even if gasoline prices continue to rise, E85 may not become more financially appealing because the price of ethanol likely will go up, too. Then again, there’s no inherent downside to buying a flexible-fuel vehicle – they still can run on gasoline, and they don’t carry the hefty premiums of a hybrid vehicle.