In a newly updated study from the think tank Resources for the Future, Shanjun Li, Joshua Linn, and Elisheba Spiller examined the program against a counterfactual world where it never existed, drawing on trends at the time from Canada. They looked at the total sales from the program (a reflection of its economic impact) and at the composition of the vehicles sold (a sign of environmental impact, based on improvements in fuel economy).

As critics of the car-rebate plan warned at the time, many of the people who took advantage of the program were on their way to buying new cars anyway. The researchers found that 45 percent of participants would have bought new cars in July and August whether the program existed or not. The rest of these people likely would have bought cars at some points later in 2009, but thanks to the promise of a $3,500-$4,500 rebate, they bought them while this deal was on the table.

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The authors did find that the program had an effect on the vehicle choices buyers made. But these results are discouraging, too. Among all of the cars bought in the U.S. in July and August, the average fuel economy was only 0.65 miles per gallon better than it would have been among newly purchased cars had the program never existed.