We estimate the rebound effect for motor vehicles, by which improved fuel efficiency causes additional travel, using a pooled cross section of US states for 1966-2001. Our model accounts for endogenous changes in fuel efficiency, distinguishes between autocorrelation and lagged effects, includes a measure of the stringency of fuel-econonty standards, and allows the rebound effect to vary with income, urbanization, and the fuel cost of driving. At sample averages of variables, our simultaneous-equations estimates of the short- and long-run rebound effect are 4.5% and 22.2 %. But rising real income caused it to diminish substantially over the period, aided by falling fuel prices. With variables at 1997-2001 levels, our estimates (ire only 2.2 % and 10. 7%, considerably smaller than values typically assumed for policy analysis. With income and starting fuel efficiency at 1997-2001 levels and fuel prices 58 percent higher, the estimates are still only 3. 1 % and 15.3 %, respectively.