Ethanol is one of the most desirable fuels for spark-ignition engines. It offers high-octane quality and a latent heat of vaporization that is four times greater than gasoline on a stoichiometric basis. Anhydrous ethanol can also readily be blended into oil-based fuels, thereby enabling improved engine efficiency and reduced greenhouse gas (GHG) emissions. However, the use of ethanol is currently constrained by low yield production processes and a reliance on considerable amounts of arable land to cultivate the most widely utilized feedstocks. These challenges could be addressed if ethanol was instead derived synthetically from petroleum-based feedstocks. This paper presents a comparative well-to-wheel assessment for three different engine-fuel systems that leverage the benefits of ethanol which has been derived synthetically and from the fermentation of biomass. In the baseline case, anhydrous ethanol (99.5% by volume) derived from corn is used to produce a high-octane E30 gasoline (RON 101). The alternative case considers synthetic hydrous ethanol (similar to 90% by volume) which is derived from direct hydration of ethene in a crude oil refinery. Hydrous ethanol is immiscible in gasoline, and is therefore utilized as a high-octane fuel for the Octane-on-Demand concept. The same engine-fuel system operated on anhydrous bioethanol is also considered for comparative purposes. Single cylinder engine tests are first used to characterize the specific fuel consumption and CO2 emissions for the different engine-fuel systems. This data is then used to construct fuel consumption maps to simulate the drive cycle fuel economy of a light-duty vehicle. Finally, the well-to-wheel GHG emissions are computed, with consequent uncertainties assessed using Monte Carlo analysis. The results demonstrate that the well-to-wheel GHG emissions for the three different engine-fuel systems are generally comparable. This is despite the Octane-on-Demand cases offering improved drive cycle fuel economy with respect to the E30 gasoline. These outcomes are shown to be largely insensitive to uncertainties in the upstream fuel production GHG emissions. Overall, this suggests that the use of synthetic ethanol in advanced engine-fuel systems could supplement bioethanol derived from first and second generation feedstocks in the future transport energy mix.