This study aims at proposing and assessing a new policy to improve fuel efficiency of cars, which obligates us to equip personal cars with a fuel consumption gauge. At first field test was done in Tokyo metropolitan area to estimate fuel economy level of auto-matic cars with the gauge. The test shows that we can save about 7.4% of total fuel consumption in case of using cars with the gauge. Based on these results, economic expenditure of fuel consumption is estimated, which is compared with the cost of the gauge. The former merit is larger than the latter cost when we use cars more than ten years. In conclusion, a policy to obligate us to equip cars with the fuel consumption gauge for improving fuel efficiency of cars is effective and available.