Fuel efficiency for cars has improved significantly in recent decades thanks to such innovations as variable-speed transmissions, efficient fuel-injection systems, and hybrid vehicles. Yet the average mileage per gallon (MPG) that autos sold in the United States achieve on the road has barely increased.

MIT economist Christopher ­Knittel explains why: because automobiles have gotten bigger and more powerful. 

Between 1980 and 2006, the average weight of vehicles sold in the United States increased 26 percent, while their horsepower rose by 107 percent. Average gas mileage increased by slightly more than 15 percent, but when the weight and power changes are accounted for, fuel efficiency increased by 60 percent, as Knittel showed in a paper in the American Economic Review. Thus if Americans today were driving cars of the size and power typical in 1980, the U.S. auto fleet would have jumped from an average of about 23 MPG to roughly 37, well above the current average of around 27 MPG.

“Most of that technological progress has gone into [compensating for] weight and horsepower,” he says. Indeed, light trucks—including SUVs—made up about 51 percent of U.S. passenger-vehicle sales in 2004, up from about 20 percent in 1980.

“I find little fault with the auto manufacturers, because there has been no incentive to put technologies into overall fuel economy,” Knittel says. Between 1980 and 2004, gas prices dropped by 30 percent when adjusted for inflation. Knittel favors a gas tax to get consumers to value fuel efficiency: “If gas prices are low, consumers are going to want big, fast cars.”