This is a guest post by Morgan Downey, author of Oil 101.

The US cash for clunkers program was designed to stimulate economic growth through the auto industry by encouraging individuals to trade in old vehicles for more fuel efficient models. The US$3 billion program ran from July 24 through August 24, 2009. The program resulted in an additional 690,114 cars being traded in. The average fuel efficiency of trade ins was 15.8 mpg and the average for the replacements was 24.9 mpg. Those receiving the clunker subsidy are supposed to be taxed on that benefit so the entire US$3 billion is not lost.

If you do the math, the saving in oil spending over the lifetime of the new vehicles could be great enough to justify the US$3 billion of taxpayer money spent. However, critics point out that the taxpayer is out of pocket for donating this one off efficiency saving to fortunate clunker owners without any benefit in return.

Now that the program is over, the data shows that there may be a single unexpected lesson.