I bought my 2006 Mercury Milan V-6 Premier with less than 25,000 on it. Rode like a dream. Recently, the vehicle miles per gallon has been falling. I have been using the same gasoline (regular unleaded) since I purchased the vehicle in September 2009. The average MPG when I bought it was 27 mpg. Today the average is 20 mpg. As bad if not worse than my 1993 Nissan Maxima, also V-6. What could be causing this problem? The gasoline in my area contains 10% ethanol year round.