A method for controlling an engine supplied with multiple fuels in which the vapor purge flow into the engine from multiple vapor storage devices each coupled to a respective, but equal number of multiple fuel tanks controlled to have the same proportion of total vapors purged as a proportion of liquid fuel delivered to the engine from said respective one of the said multiple fuel tanks. The method includes increasing the delivery of fuel from one of the multiple fuel tanks containing fuel with the highest-octane rating of all the fuel tanks when the vapor storage canister coupled to the fuel tank with the highest-octane fuel is not being purged of its fuel vapors. Additionally, the method further comprises a feedback control responsive to an exhaust gas oxygen sensor to adjust said fuel delivered from said multiple fuel tanks to the engine to maintain engine air-fuel ratio around stoichiometry.