A fuel tank system for gasoline or flexible gasoline/ethanol powered vehicles that use independently controlled direct ethanol injection to provide a large on-demand octane boost is disclosed. The on-demand octane boost is used when needed to prevent knock. The ethanol can be in the form of 100% ethanol or E85 (a 85% ethanol, 15% gasoline mixture) and is stored in a second tank that is separate from the tank that which contains the primary fuel. The primary fuel can be gasoline, E85, ethanol or a mix of these fuels. The fuel tank system enables convenient, quick, flexible and minimal cost refueling of the separate fuel tank. A range of fueling options is available to provide the driver with the maximum freedom to choose fuels depending upon price and availability. Valves may be utilized to direct the flow in fuel to the various tanks.