ITS Spot, a 5.8 GHz DSRC system, has begun operation in Japan in 2011 and provides dynamic route guidance, driving safety support, and ETC system. Some of the expected effects of this service are smoother traffics, fewer traffic accidents, and environmental impact reduction. Evaluating this last effect will require accurate estimates of carbon dioxide emissions, but existing estimation methods are insufficiently accurate for application to ITS Spot. In recent years, new OBD-II connectors to provide information on factors such as fuel consumption have become commercially available, and the additional information they provide could make estimations of carbon dioxide emissions possible. Here, we use such automobile information to identify factors for adjustment of existing methods of carbon dioxide emission estimation, thereby developing a high-precision estimation tool for evaluating the effect of environmental impact reduction.