A fuel system for improved fuel efficiency which can be contained in a single fuel source, such as a fuel tank of a vehicle, having a gasoline phase comprising gasoline or gasohol; and an anti-knock phase comprising a glycol anti-knock subagent, water and one or more of a second anti-knock subagent selected from the group of methanol, ethanol and mixtures thereof; such that the anti-knock agent phase is substantially immiscible with the gasoline phase.