This paper clarifies influence rate of traffic-flow and eco-driving factors that have effect on on-road fuel economy and a case study was conducted to estimate the CO₂ reduction potential due to traffic-flow smoothing and eco-driving promotion by analyzing floating car data from throughout Japan. The data employed in the study was obtained from hybrid vehicles equipped with an Eco Assist system. Previous research has reported that repeated use of these vehicles enhances fuel economy by approximately 10%. First, multiple regression analysis was performed on the subject floating car data to obtain a polynomial with fuel economy as the explained variable and items related to traffic flow and eco-driving as the explanatory variables. Average travel speed was found to have the greatest effect on fuel economy. In order to calculate the potential reduction in CO₂ emissions, the distributions of the explanatory variables were set for present conditions and assumed conditions of smoother traffic flow and more eco-driving promotion. These assumed conditions were applied to the polynomial and the CO₂ reduction potential was calculated. The assumed conditions were calculated to have a fuel economy enhancement potential of 18%, mainly due to traffic-flow smoothing. On the other hand, eco-driving was calculated to have fuel economy enhancement potential of 1% over and above the approximately 10% enhancement of Eco Assist. In addition, it was shown that two-thirds of the potential reduction in CO₂ emissions was contributed by improvements at travel speeds of 30 km/h and below.