In 2017, the Chinese government issued a strategic policy of nationwide use of bioethanol as a gasoline-blending component by 2020 for the consideration of reducing smog and greenhouse gas (GHG) emissions. It is highly relevant to estimate the benefits of well-to-wheel (WTW) GHG emission savings using future engine technologies. However, literature about the WTW GHG emissions for ethanol blends did not cover the engine efficiency gains in engines with future technologies. In a previous publication from the authors’ group, an empirical model was developed to predict the anti-knock property and engine thermal efficiency gains of ethanol blends in spark-ignition (SI) engines. This paper is a follow-up study, looking at not only the potential engine thermal efficiency gains but also WTW GHG emissions in future engine technologies. More specifically, a case study of adding bioethanol to two representative E10 fuels (main- and premium-octane-grade fuels) from China was conducted. It is assumed that future engine technologies enable an adjustable compression ratio (CR) according to the octane rating of ethanol blends, allowing for the maximum extraction of the benefit of a high anti-knock property of ethanol blends. In addition, the sensitivity of GHG intensity of bioethanol on WTW GHG emissions is analyzed and discussed. It is found that the chemical and cooling effects of ethanol blends are the dominant factors contributing to engine thermal efficiency gains. For the ethanol blends with the RON84.5 base gasoline, the negative impact of a lower heating value (LHV) of ethanol blends on the vehicle mileage range can be completely offset by the engine thermal efficiency gain, enabled by a higher octane rating of ethanol blends. Assuming that, in China, in the future, bioethanol has a GHG intensity of 33 g of CO2 equiv/MJ (grams of CO2 equivalent per megajoules of LHV) in comparison to E10, E30 led to a 21.2% reduction of WTW GHG emissions in a turbo-charged (TC) direct-injection spark-ignition (DISI) vehicle. Among this 21.2% reduction, one-third is due to the thermal efficiency gain and two-thirds is due to the use of renewable bioethanol. Reducing the GHG intensity of bioethanol is a key to lowing WTW GHG emissions. For the TC DISI engine technology, when E10 is used as the baseline fuel, every 1 g of CO2 equiv/MJ reduction in GHG intensity of bioethanol leads to a 0.239 g of CO2 equiv/MJ of WTW GHG emission saving for vehicles fueled with E20.