This paper explores the potential for reducing transport-related greenhouse gas (GHG) emissions by introducing high-efficiency spark-ignition engines with a dual-fuel injection system to customize the octane of the fuels based on real-time engine requirements. It is assumed that a vehicle was equipped with two fuel tanks and two injection systems; one port fuel injection and one direct injection line separately. Each tank carried low octane and high octane fuel so that real-time octane blending was occurred in the combustion chamber when needed (Octane On-Demand: OOD). A refinery naphtha was selected for low octane fuel (RON=61), because of its similarity to gasoline properties but a less processed, easier to produce without changing a refinery configuration. Three oxygenates were used for high octane knock-resistant fuels in a direct injection line: methanol, MTBE, and ETBE. Single cylinder engine tests were conducted to obtain a specific fuel consumption map and an optimum octane map in each dual fuel combination. Tank-to-wheel vehicle fuel consumption was estimated using a vehicle simulation tool under four driving cycles (FTP city and highway, WLTP, and US06), to cover a wide range of driving circumstances. For the vehicle with a naturally aspirated 4-cylinder engine, total dual fuel consumption were improved over baseline gasoline single fuel up to 4% reduction in mass with all driving cycles except for US06. More importantly, only less than 15% oxygenate was required for those three cycles. As a result, blending RON was only in the range of 66∼73, significantly below current gasoline octane levels in the market. A Well-to-Wheels (W-t-W) GHG emissions assessment was conducted to estimate the overall emissions benefits of the dual fuel system. W-t-W carbon intensity was reduced up to 18% relative to the single gasoline fuel system when crude naphtha is used as a low octane fuel. However, if a pathway for naphtha and oxygenate are chosen different than petroleum source, for example, natural gas feed Fischer-Tropsch naphtha and coal base methanol, GHG emissions are worse than baseline case. When the dual fuel system was combined with downsized boosted engine technology, synergic benefit was obtained maximum 30% W-t-W CO2eq reduction (15% from downsizing, 10% from mitigating knock via a high octane component and 5% from minimizing octane upgrade process in the refinery). Therefore, a suggested OOD system can be a potential mobility solution for a future fuel and engine system. It not only provides a desirable high octane quality fuel for a high boost and high compression ratio engine, but also mitigates a revolutionary change in the refinery with a low octane naphtha fuel.