It is becoming increasingly clear that research into alternative fuels, including drop-in fuels, is essential for the continued survival of the internal combustion engine. In this study, the authors have evaluated olefinic and oxygenated fuels as drop-in fuels using a single-cylinder engine and considering fuel characteristic parameters. The authors have assessed thermal efficiency by adding the EGR or air from 0 to the maximum value that allows stable combustion. Next, we attempted to predict fuel efficiency for four types of passenger cars (Japanese small K-car N/A, K-car T/C, Series-HV, and Power-split-HV) by changing the fuels. We created a model in OpenModelica to estimate fuel efficiency during WLTC driving. The results indicated that fuel economy could potentially be improved by adding an olefin fuel that burns stably even with a large amount of EGR or air and an oxygen fuel whose octane number increases. It was observed that the fuel economy improvement rate was particularly notable for Series-Hybrid Vehicle (HV) with operating under specific load and engine speed conditions.