A flex fuel vehicle is one with an engine that is capable of running on a varied mixture of fuels, typically gasoline and ethanol. Vehicles with multiple fuel systems — such as might run on hydrogen cells and gasoline, for instance — are called bi-fuel or dual fuel vehicles. To be technical, all automobiles that can take a mix of gas and ethanol without modification can be described using this term, and most cars on the road today can. However, a true flex fuel vehicle can go from 100% gasoline to 100% ethanol and back.
A sensor in the fuel system of a flex fuel vehicle measures the relative proportions of the two possible fuels and automatically adjusts the "tuning" of the car's engine so that the mixture, whatever it is, will burn cleanly. This sensor and self-tuning capability are what makes this type of vehicles possible, and they have been around for a number of years. The oil crisis of the 1970s spurred interest in and research on alternative fuels, and one of the leading candidates has always been ethanol, a plant-based alcohol that can be made from domestic farm products. Until a few years ago, drivers in the US could only buy SUVs that worked with gas or ethanol, but car manufacturers are marketing vehicles in other classes, such as sedans and wagons.
Brazil leads the US — and the world — in the adoption of ethanol as an alternate fuel source and consequently in the production and use of vehicles that can run on it. Brazil's government has poured millions into researching alternate fuels and transition from gasoline dependence, and today produces ethanol from locally grown sugar cane. Cars sold in Brazil must be able to take at least a 25% ethanol to 75% gasoline mixture. Cars that can use both types of fuel are much more prevalent in Brazil than in the US, and a wider variety of classes, down to subcompacts, are sold there. Today, over half the cars sold in Brazil are flex fuel vehicles.