A method for reducing required octane number and a spark ignition gasoline engine system with hydrogen-enhanced knock resistance. The method for reducing required octane number of gasoline needed to prevent knock includes the addition of hydrogen or hydrogen-rich gas containing carbon monoxide to gasoline. Octane number can be improved by 5 or more for a hydrogen energy fraction of 10%. The spark ignition gasoline engine system includes a spark ignition gasoline engine and a source of gasoline and hydrogen or hydrogen-rich gas. Apparatus is provided to supply the gasoline and the hydrogen or hydrogen-rich gas to the engine at a varying hydrogen or hydrogen-rich gas to gasoline ratio selected both to prevent knock and to ensure a desired level of combustion stability throughout a full range of engine operation. The engine system may be normally aspirated or boosted; the compression ratio may be high such as greater than 11 or below 11, and EGR may be added. The hydrogen or hydrogen-rich gas to gasoline ratio may be controlled as a function of boost pressure, torque, engine speed, or air/fuel mixture ratio.