It is known that driving style is highly correlated with safety and fuel economy. In general, drivers who have moderate driving style shows safer driving behavior than aggressive drivers, and they may achieve higher fuel efficiency than their aggressive counterpart. Some previous studies found forward collision warning systems supported to change driving style toward safer behavior. However, their potential benefit of improving fuel economy was not clearly evaluated. This paper aims to analyze the effect of the advanced driver assistance systems (ADAS) on driving style and fuel economy using field operational test data from 52 participants. A half of participants were supported by ADAS and the others were not supported. Each participant drove approximately 5.5 km of rural road (about 10 minutes), 6.2 km of urban road (about 25 minutes) and 9.6 km of highway (about 10 minutes). The results suggested that there has been significant interaction between ADAS and gender on driving style, and consequently ADAS supported younger male drivers’ improvement in fuel efficiency was observed.