CAMBRIDGE, MA – A study by MIT economist Christopher Knittel found that although the automakers were able to improve the fuel efficiency of their vehicles’ engines between 1980 and 2006, increased vehicle weight led to stagnation of mpg numbers during that period. Knittel conducted his study by using data from NHTSA, auto manufacturers, and trade journals.

Knittel found that between 1980 and 2006, the average gas mileage of vehicles sold in the United States increased by slightly more than 15%. During that same period, though, average curb weight of those vehicles increased 26%, and average horsepower rose 107%. All factors being equal, fuel economy actually went up by 60% during that period.

MIT’s report said that if Americans today were driving cars of the same size and power that were typical in 1980, average mpg would be 37 rather than the current average of 27.

“Most of that technological progress has gone into [compensating for] weight and horsepower,” Knittel said.

In addition, the vehicle types available also affected fuel economy. In 1980, light trucks represented about 20% of passenger vehicles sold in the U.S., according to the study. By 2004, light trucks and SUVs made up 51% of passenger-vehicle sales.

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

Knittel noted that between 1980 and 2004, gas prices dropped by 30% when adjusted for inflation.

Knittel’s study found that the automakers could meet the new CAFE standards by maintaining the rate of technological innovation experienced since 1980 while reducing the weight and horsepower of the average vehicle sold by 25%. He also said that reducing vehicle weight and power to that of the average seen in 1980, along with continuing the trend of improving fuel economy, would lead to a fleet-wide average of 52 mpg by 2020.

Despite these conclusions, Knittel said he’s skeptical that CAFE standards alone would have as great an impact as a gas tax. He said mileage regulations “end up reducing the cost of driving. If you force people to buy more fuel-efficient cars through CAFE standards, you actually get what’s called ‘rebound,’ and they drive more than they would have.” He said a gas tax would produce less of a “rebound” effect.