From an article in today's Wall Street Journal:
Q: Since most if not all of the gasoline sold in the United States is mixed with 10% ethanol, which degrades gas mileage, does the EPA take this into consideration when determining average gas mileage?
—J. Kamisar, Weston, Conn.
A: No, the Environmental Protection Agency bases its fuel-economy ratings for vehicles on 100% gasoline. Drivers using the typical 10% ethanol blend should expect a decrease of 3% to 4% in fuel economy.
If this is true, it's outrageous. The government's fuel economy ratings are for a fuel that's different from the fuel that the government forces drivers to use.