An actual fuel-consumption (FC) database with hierarchic structure was established based upon voluntary reported fuel-consumption log data collected through an internet-connected mobile-phone system. The current status of the actual FC in Japan has been investigated by the database and CO2 emissions from petrol-fuelled passenger vehicles (PVs) were estimated. It was confirmed that there lies a statistically-significant difference between actual FC and Japanese 10.15 mode FC and that the actual FCs differ by geographical and social aspects. In addition, the estimated actual FC-based CO2 emissions were consistent with the present CO2 emissions. Since a large amount of CO2 emissions mitigation in the transportation sector is expected by improving the FC of motorcars, the following items are required to significantly reduce the CO2 emissions in the motorcar sector: (1) comprehensive measures to reduce CO2 emissions regarding actual FC, (2) R&D of motorcars with high actual FC and (3) endorsement and execution of environmentally-friendly driving to improve actual FC.