Higher octane gasoline will be an important factor in enabling future spark-ignition engines to meet increasingly stringent fuel economy and CO2 emissions requirements. The most effective method to raise the octane 'floor' of regular grade gasoline is through the use high octane blend components, such as methanol and ethanol. However, this is often limited by the negative effects associated with energy density, phase separation and cold engine starting. This paper therefore examines the optimal way to leverage the most widely available high octane fuels to improve the performance and environmental impact of light-duty vehicles. A comprehensive set of baseline engine data is first presented for two splash-blended gasolines containing ethanol (E10 and E30). The octane quality of these fuels (RON 93 and 101) has been raised by directly displacing the gasoline blendstock (RON 90) with higher octane ethanol (RON similar to 109). The two splash-blended gasolines are compared with the Octane-on-Demand concept, which instead leverages only the necessary amount of high octane fuel when the octane requirement of the engine exceeds the level that can be provided by the oil-derived base fuel. The same gasoline blendstock is used in both cases, thus enabling the leveraging effect of the high octane fuels in the Octane-on-Demand configuration to be directly quantified. The results demonstrate that the Octane-on-Demand concept used in conjunction with either methanol or ethanol provides comparable or lower specific CO2 emissions to the E30 gasoline, with up to a 10% improvement in specific fuel consumption. The use of a non-traditional engine calibration strategy that maximizes the trade-off between thermal efficiency and fuel energy density also enables the amount of high octane fuel required to suppress knock to be reduced by at least 25%, with methanol offering the greatest benefits. This however comes at the expense of marginally higher specific CO2 emissions than could otherwise be achieved. Overall, this work suggests that powertrains designed around the Octane-on-Demand concept may provide greater social and environmental benefits than those designed for high octane splash-blended gasolines with significant methanol or ethanol content. (C) 2017 Elsevier Ltd. All rights reserved.