Diffusion of battery electric vehicles and fuel cell hybrid electric vehicles can contribute to reduce passenger light-duty vehicle fleet CO2 emissions. However, barriers such as higher vehicle capital cost and lack of electricity and hydrogen infrastructure prevent their deployment. A vehicle stock turnover model was used to assess the impact of mini-sized and zero-emission vehicle diffusion on passenger light-duty vehicle fleet energy and material consumption, CO2 emissions and cost, focusing on Japan. 2050 passenger light-duty vehicle fleet energy consumption and tank-to-wheel CO2 emissions in the base scenario are 48.7 and 51.9% lower than the 2012 values. Diffusion of mini-sized and battery electric vehicles provides the largest energy consumption and CO2 emissions reductions, 64.7, and 87.8% compared with the 2050 baseline values. Incremental cost of zero emission vehicles is reduced through downsizing. The 2050 net cash flow for battery electric vehicles diffusion is reduced from 15.9 to −16.7billion USD/year if downsizing is applied; while in the case of fuel cell hybrid electric vehicle diffusion, downsizing reduces the 2050 net cash flow from −12.5 to −47.8billion USD/year. Thus, shifting to mini-sized zero emission vehicles provides the quadruple benefit of reducing energy and material consumption, CO2 emissions and cost.