My mom's 1981 Chevrolet Citation got 24 miles to the gallon in the city and 36 mpg on the highway. The 2012 Chevy Sonic hatchback gets 25 mpg in the city and 35 on the highway.

Three decades later, and fuel economy hasn't improved.

Actually, that's not quite true, MIT economist Christopher Knittel wrote in a new paper, "Automobiles on Steroids," just published in the American Economic Review.

Fuel economy actually increased by 60 percent between 1980 and 2006, all things being equal. The problem is, all things weren't kept equal.

"Most of that technological progress has gone into weight and horsepower," Knittel said in an MIT article on his paper.

Automakers boosted average vehicle weight by an average of 26 percent and horsepower by 107 percent. So actual average gas mileage of vehicles sold in the U.S. rose by just 15 percent.

If we were driving the typical 1980-size and -performance car around, the country's fleet would average 37 mpg, up from 23 mpg in 1980 and the 27 mpg we actually get now.

More recently, automakers have responded to higher fuel prices and changing customer tastes with smaller, more-efficient cars, such as the Sonic, which is smaller than the Citation.

But there's still the performance issue. The Citation had an 84-horsepower engine and went from zero to 60 mph in 16.1 seconds. The Sonic: 138 horsepower, zero to 60 in 8.2 seconds.

Who is to blame? We all are, according to Knittel.

"I find little fault with the auto manufacturers, because there has been no incentive to put technologies into overall fuel economy," he said. "Firms are going to give consumers what they want, and if gas prices are low, consumers are going to want big, fast cars."

Between 1980 and 2004, gas prices dropped by 30 percent when adjusted for inflation. This means the way to get people, including those who build cars, to focus on fuel efficiency is higher fuel prices.

"When it comes to climate change, leaving the market alone isn't going to lead to the efficient outcome," Knittel said. "The right starting point is a gas tax."

The Obama administration last year announced an increase in automaker Corporate Average Fuel Economy standards to 35.5 mpg by 2016 and 54.5 mpg by 2025. While that will spur more efficient cars, people who buy those cars will drive more if fuel prices remain the same, according to Knittel.

A higher gas tax, on the other hand, would bring more efficient vehicles and less driving.
