The Environmental Protection Agency, National Highway Traffic Safety Administration, and California Air Resources Board released the joint mid-term Technical Assessment Review of the light-duty GHG standards in July of 2016. The review generally asserted that the GHG standards adopted in calendar year 2012 for 2022-2025 model year vehicles were feasible. Although many different technologies were evaluated, the review did not assess the benefits of high compression ratio engines enabled by a high-octane low carbon fuel. This study fills in the gap in the Technical Assessment Review by examining the impacts of a 98-research octane number gasoline-ethanol blend with 25 percent ethanol. We find that this fuel would enable higher compression ratios to improve tailpipe greenhouse gas emissions by about 6 percent on most engines. We also find that this option might cost approximately $100 per vehicle, but that there would be fuel cost savings of about the same amount over the life of a vehicle, since the 98 octane 25% ethanol blend would be a few cents lower in cost per gallon than today’s 10% ethanol blend. We assume the same tailpipe emission standards as the Review. When we use EPA’s OMEGA modeling system to evaluate this option, adding in only the high octane high compression ratio and leaving everything else in the modeling essentially the same, we find that the cost of meeting the model year 2025 greenhouse gas emission standards is reduced on a national basis from 23.4 billion dollars to 16.4 billion dollars. Furthermore, overall greenhouse gas emission benefits are higher than the EPA’s proposal without a high-octane fuel, because of the lower lifecycle greenhouse gas emissions of the additional ethanol.