Since 1978 new vehicle fuel economy has changed from year to year quite differently for automobiles and light trucks. Automobiles experienced three years in which MPG gains were quite large, whereas light trucks experienced only one such year. The MPG changes and causes for these changes are analyzed for autos and light trucks. The contribution of consumer actions are analyzed.The consumer actions collectively accounted for only 28 percent of the 6.7 MPG gain for automobiles, for 63 percent of the 2.7 MPG gain for light trucks during the 1978-1984 period. Shifts across size classes accounted for only 7.8 percent of all auto MPG gains and for 21.9 percent of all light truck MPG gains.