In The ethanol myth, Consumer Reports shares its findings from its test of a flexible-fuel Chevy Tahoe:
When running on E85 there was no significant change in acceleration. Fuel economy, however, dropped across the board. In highway driving, gas mileage decreased from 21 to 15 mpg; in city driving, it dropped from 9 to 7 mpg.You could expect a similar decrease in gas mileage in any current FFV. That’s because ethanol has a lower energy content than gasoline: 75,670 British thermal units per gallon instead of 115,400, according to the National Highway Traffic Safety Administration. So you have to burn more fuel to generate the same amount of energy. In addition, FFV engines are designed to run more efficiently on gasoline. E85 fuel economy could approach that of gasoline if manufacturers optimized engines for that fuel.
When we took our Tahoe to a state-certified emissions-test facility in Connecticut and had a standard emissions test performed, we found a significant decrease in smog-forming oxides of nitrogen when using E85. Ethanol, however, emits acetaldehyde, a probable carcinogen and something that standard emissions-testing equipment is not designed to measure.