E15 (15% ethanol, 85% gasoline) cannot be used legally in standard (non-flexible fuel) vehicles. For this to happen, the U.S. Environmental Protection Agency (EPA) must approve a waiver to the Clean Air Act, classifying the blends as "substantially similar" to gasoline. -US Department of Energy, February 19, 2010

The U.S. Environmental Protection Agency (EPA) today waived a limitation on selling gasoline that contains more than 10 percent ethanol for model year 2001 and later passenger vehicles, SUVs, and light pickup trucks. The waiver applies to fuel that contains up to 15 percent ethanol – known as E15. “Recently completed testing and data analysis show that E15 does not harm emissions control equipment in newer cars and light trucks,” said EPA Administrator Lisa P. Jackson. “Wherever sound science and the law support steps to allow more home-grown fuels in America’s vehicles, this administration takes those steps.” -US EPA, January 21, 2011

What Vehicles and Engines May Not Use E15? -US EPA, June 23, 2011
   All motorcycles
   All vehicles with heavy-duty engines, such as school buses, transit buses, and delivery trucks
   All off-road vehicles, such as boats and snowmobiles
   All engines in off-road equipment, such as lawnmowers and chain saws
   All MY2000 and older cars, light-duty trucks, and medium-duty passenger vehicles (SUVs)