Input- and output-split hybrids using a single planetary gear (PG) can provide high fuel economy, but they tend to suffer from low acceleration performance. In order to improve their acceleration performance, speed reduction (multiplication) gears (SRG/SMG) have often been employed in various mass-produced split hybrids. In fact, adding one SRG (SMG) to input- or output-split hybrids can improve not only the acceleration performance, but also the fuel economy. Nevertheless, the full potentials of using SRGs (SMGs) have not yet been thoroughly investigated because the design space of input- and output-split configurations using one SRG (SMG) is huge; 432 configurations can be generated using two PGs where one PG is used as an SRG/SMG. Thus, in order to investigate the impacts of SRG (SMG) within a reasonable time, an efficient analysis procedure is required. In this paper, SRG (SMG) is added in front of each component of two representative input- and output-split HEV (Prius 2nd and Volt 1st) configurations. The fuel economy and the acceleration performance of those configurations are assessed within a virtual design space, to eliminate redundant designs. Lastly, in order to investigate the impact of SRG (SMG), the performance metrics are plotted onto a performance map and grouped together by each 1PG + SRG (SMG) configuration. The results show that adding an engine SRG to the Prius 2nd generation configuration can dramatically enhance the acceleration performance, and adding an engine SMG to the Volt 1st generation can significantly improve the fuel economy of the output-split configuration.