The aim of this study is to quantify the potential reduction of CO2 emissions by passenger vehicles over the long term through the introduction of compact cities. We determined the correlation between population distribution and passenger car CO2 emissions from 1980 to 2005 and simulated passenger car CO2 emissions in 2030 under both compact and dispersed scenarios. We conducted correlation analysis and scenario analysis with the data sets of municipal CO2 emissions of passenger cars, national population census figures, and future population distribution scenarios. Then, we estimated the annual CO2 emissions of passenger cars per capita by mesh cell density category. The results show that the difference in emissions per capita between the compact and dispersed scenarios is roughly 5% in Japanese municipalities as a whole.