If you live in the United States and value fuel efficiency, you might catch yourself looking longingly overseas at super efficient vehicle fleets and wondering, why not us? One MIT researcher looked into the predicament and found that though it might not look like we are making strides, we are. The average, fuel efficiency for US vehicles actually increased by 60 percent between 1980 and 2006. The problem is that cars in the US got bigger (by 26% on average) and their horsepower increased (by 107 hp on average), which, when factored in, means that the average fuel efficiency of American cars only increased by a mere 15%. Almost all of the new technology went into making cars more efficient per pound of weight so that the cars could get bigger and still fit within average mile per gallon expectations.

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I find little fault with the auto manufacturers, because there has been no incentive to put technologies into overall fuel economy,” said MIT economist Christopher Knittel, author of the study. “Firms are going to give consumers what they want, and if gas prices are low, consumers are going to want big, fast cars.” Knittel found that when adjusted for inflation, the price of gas dropped 30% between 1980 and 2004.

Knittel used numbers from the National Highway Traffic Safety Administration, auto makers and trade journals to compile his research. He found that in 1980 light trucks were just 20% of the US vehicle fleet. In 2004 light trucks — with the addition of SUVs — made up 51% of the fleet. Knittel punctuated his paper with a strong opinion that in order to use those advances in fuel efficiency to decrease gas use and emissions we’ll need the government to step in and do some friendly nudging. “When it comes to climate change, leaving the market alone isn’t going to lead to the efficient outcome,” Knittel says. “The right starting point is a gas tax.”