Automobiles and light trucks are important sources of greenhouse gases, particularly carbon dioxide. Continued fuel economy improvements for these vehicles, using available, cost-effective, marketable fuel economy technology can hold CO2 emissions constant through the year 2000. An aggressive effort to improve fuel economy through 2001 could hold CO2 emissions in 2015 to near present levels. CO2 emissions can be reduced by controlling the growth of vehicle travel, but this is likely to be a very costly strategy. Recent trends, such as declining new car MPG, increasing market share of light trucks, and a growing gap between federal test MPG and on-road performance, indicate that a 50% increase in light duty vehicle CO2 emissions by 2015 is not out of the question.