A: Flex fuel is a fuel mixture of ethanol and gasoline. The most common type is E85, a blend of 85% ethanol and 15% gasoline, though other varieties exist. Ethanol is made from renewable sources such as corn, sugar cane, and other grains, so it rose to popularity as a means to reduce oil dependence; however, studies debate its energy efficiency (meaning it may require more ethanol to go the same distance as gasoline).