Flex fuel, short for flexible fuel, refers to an automotive drivetrain that can run on either traditional or ethanol-blended gasoline. Flexible fuel vehicles (FFVs) are designed to run on gasoline or gasoline-ethanol blends of up to 85% ethanol (E85) . FFVs have one fuel system, and most components are the same as those found in a conventional gasoline-only car. Some special ethanol-compatible components are required to compensate for the different chemical properties and energy content in ethanol, such as modifications to the fuel pump and fuel injection system. The engine control module (ECM) is also calibrated to accommodate the higher oxygen content of ethanol. Modern flex-fuel engines are capable of burning any proportion of the resulting blend in the combustion chamber as fuel injection and spark timing are adjusted automatically according to the actual blend detected by a fuel composition sensor.

The benefits of using flex fuel include reduced greenhouse gas emissions and fewer toxic fumes pumped into the environment. However, the drawbacks to using E85 include reduced fuel efficiency, with flex-fuel vehicles returning mileage that is between 15 and 27 percent worse than a regular gas-fueled engine because of the lower energy content in the fuel.

FFVs are factory-made and are capable of operating on gasoline and gasoline-ethanol blends, and many vehicle owners may not realize that their car is an FFV and that they have a choice of fuels to use. To identify an FFV, one can check the owners manual or look for badges on the vehicles body with terms such as "E85," "Flex-Fuel," or "FFV".