Critics of ethanol often allege that because ethanol contains fewer British Thermal Units (BTUs) of energy (83,263 BTU's/gal ethanol vs. 114,132 BTU's/gal gasoline), ethanol-blended fuel has a negative impact on gas mileage.
In reality, variables such as speed, stop-and-go driving, tire pressure, air conditioner use, and the weather's effect on driving conditions have a much greater impact on fuel economy than what fuel you use in your engine. In fact, some carbureted vehicles that run rich experience an increase in fuel economy with ethanol-blended fuels. Other tests and studies suggest that fuel economy may decrease by approximately 2% in fuel-injected cars, such that a car averaging 30 miles per gallon (MPG) on the highway would average 29.4 MPG using an ethanol-blended fuel (10% ethanol), not enough to be detected by the average driver.
For E-85, it has a much higher octane (ranging from 100 to 105) than gasoline. Flexible fuel vehicles (FFVs) are not optimized to E-85, so they experience a 10% to 15% drop in fuel economy (Source: National Ethanol Vehicle Coalition). This will vary based on temperature and driving conditions. At the same time, the use of ethanol contributes to a cleaner environment, stronger economy, and increased energy security.