In the face of drought, rising food prices, lower fuel economy and easily recovered domestic oul, the EPA will be requiring gas stations to begin vending E15 soon. In addition to all the above reasons not to burn ethanol as a fuel in the first place, there is the potential for damage to older engines, and the added feature of the ethanol suddenly falling out of solution when condensate water gets mixed into the fuel.
That last bit doesn't happen around here where 30% humidity is considered oppressively muggy, but I'm told it regularly reaches higher levels in other parts of the country.
The EPA's clever solution is to set a minimum sale of 4 gallons of E10 from pumps where the hose is common to multiple grades of fuel.
Most people I know fill a 15 gallon tank when it gets down to 1/4 full, so this shouldn't be much of a problem. I already deal with this when I fill my motorcycle, which prefers premium. I wait until the 6-gallon tank is down to 1-2 gallons, then top off. Figuring the fuel line at the pump runs from the bottom of the pump, where the grade is switched, to the top, then back down the hose, this is about 5 yards of 1" ID hose containing about .6 gallons of whatever the fellow before me used, probably sub-grade rotgut regular. I figure my 4-gallons of premium is 85% good stuff and 15% swill.
Happy motoring tip: Fill up in the better neighborhoods. Check out the clientele at the pumps when you pull up. Get in behind the fellow filling his 7xx BMW and wait for him to finish. You get better gas that way.
Of course if you don't have an upscale neighborhood handy, just remember, the hose only holds .6 gallon. You can pump this out on the pavement first then top off your tank with 100% good stuff. If the station owner or the other customers object, tell them the EPA made you do it.