E85 flex-fuel vehicles are cars and trucks capable of running on either standard gasoline or a blend of gasoline and alcohol known as E85. These vehicles have had modifications to their engine and fuel systems to accommodate E85 fuel, which contains between 51 and 83 percent ethanol.
While many standard automobiles are compatible with low-ethanol alcohol and gasoline fuels such as the E10 blend commonly sold at gas stations, as of 2015, most cars cannot run on E85 due to its high alcohol content. High alcohol levels in fuel can cause corrosion and remove lubricants in standard engines and fuel systems, while the greater flammability of ethanol as compared to gasoline makes it dangerous for use in vehicles with standard fuel systems. Flex-fuel vehicles have engines and fuel systems modified to resist the corrosive effects of ethanol and to prevent unwanted combustion of high-alcohol fuels such as E85. The engine control units used in flex-fuel vehicles can also sense the type of fuel in use to optimize the engine's performance for regular gasoline or E85.
Flex-fuel vehicles operate almost indistinguishably from normal vehicles when running on gasoline. However, a significant reduction in gas mileage usually occurs when using E85. This reduction varies depending on the vehicle and the specific ethanol content of the E85 used, but can be as high as 33 percent in some cases.