The gap between on-road fuel efficiency and official fuel efficiency of passenger vehicles reported by manufacturers is widening, which would lead to the underestimate of real fuel consumption and biased projection of energy demand in the road transport sector. To investigate the gap between official and on-road fuel efficiency of passenger vehicles, we first built a comprehensive dataset of 393,108 valid observations, including vehicle system parameters, fuel-efficient technology, etc. Then we developed two regression models to quantify the driving factors of the increasing gap and slow improvement in on-road fuel efficiency, and the contribution of each factor to the gap. Results show that the fuel efficiency gap enlarged from 14.4% in 2010 to 25.1% in 2016, and the average on-road fuel efficiency did not have substantial improvement though the official fuel efficiency did. The fuel-efficient technologies have the greatest effect on both the fuel efficiency gap and on-road fuel efficiency among the factors considered in this study. Among the fuel-efficient technologies, hybrid electric vehicle would decrease the ratio of on-road to official fuel efficiency by 12.4% and improve the on-road fuel efficiency by 44.1%. Impacts of other fuel-efficient technologies were also assessed. Further government effort on policies to encourage fuel-efficient technologies for improving real on-road fuel efficiency is needed.