Source: Light-Duty Automotive Technology and Fuel Economy Trends. http://www.epa.gov/otaq/fetrends.htm

Average Gasoline Consumption for New Vehicles, United States, 1972-2009 (in miles per gallon)

In 1975, after the First Oil Shock, the US Congress authorized the Department of Transportation to set and enforce new automobile efficiency standards known as CAFE (Corporate Average Fuel Economy). The fuel efficiency of new vehicles increased substantially between 1975 and 1988, in part because of the legislation but also in part of new imported fuel efficient vehicles, namely from Japan. After 1988, the fuel efficiency of new vehicles leveled off at about 28.5 miles per gallon. Today, CAFE standards are 27.5 mpg for passenger cars and 20.7 mpg for light trucks. From the late 1980's, light trucks, particularly Sports Utility Vehicles, gained market share and began to have a negative impact on the average fuel efficiency. In 2007, the CAFE standards for light trucks were increased from 20.7 mpg to 22.2 mpg which resulted in an improvement in fuel economy. Rising oil prices have also played a substantial role.