The flex-fuel system works by measuring the concentration of oxygen in the exhaust gas passing out of the engine and thus enabling the concentration of ethanol in the fuel to be optimized so that amounts of fuel injected and the ignition spark timing can be perfected. The end result is that the engine can handle gasoline-ethanol fuel mixtures with proportions of ethanol between 0% and 100%. Final performance and fuel-economy is said to be comparable with regular gasoline vehicles.