A flexible fueled vehicle (FFV) has a single fuel tank, fuel system, and engine. The vehicle is designed to run on unleaded gasoline and an alcohol fuel (usually ethanol) in any mixture. The engine and fuel system in a flex-fuel vehicle must be adapted slightly to run on alcohol fuels because they are corrosive. There must also be a special sensor in the fuel line to analyze the fuel mixture and control the fuel injection and timing to adjust for different fuel compositions. The flex-fuel vehicle offers its owner an environmentally beneficial option whenever the alternative fuel is available.

Flex-fuel technology was created by Ford Motor Company in the mid-1980s. Flexible fueled vehicles (also called variable fuel vehicles) have been produced by Ford (Ranger, Crown Victoria and Taurus), GM (Chevy S-10 and GMC Sonoma), and Daimler-Chrysler (Plymouth Voyager and Dodge Caravan).