A fuel control for an engine using a liquid fuel mixture of two fuels such as gasoline and methanol acts in closed loop on a vehicle exhaust mounted oxygen sensor to maintain a stoichiometric air/fuel ratio. The closed loop portion of the control compares the oxygen sensor output voltage to a reference or reference window in the process of indicating a lean or rich mixture; and the fuel control is responsive to a fuel composition sensor to shift the reference or reference window with changing fuel composition to maintain a consistent relationship between the reference or reference window and stoichiometry as the fuel composition changes.