Rick Larrick, the Duke business professor who has come up with a new formula for measuring fuel efficiency -- he looks at consumption, in gallons per 100 miles -- has posted an analysis of the results so far under cash for clunkers. With DOT reporting that clunkers traded in averaged 15.8 miles per gallon, compared with 25.4 miles per gallon for the new vehicles, here's a summary of Larrick's analysis:

Let's look at the gas savings created by replacing a fleet that has an average MPG of 15.8 with a fleet that has an average MPG of 25.4 using gallons per 100 miles:

100/15.8 = 6.33 gallons per 100 miles
100/25.4 = 3.94 gallons per 100 miles

That's a savings of 2.4 gallons per 100 miles. If we take into account a rebound effect (increased driver in new, efficient vehicles), that might reduce the net gas savings to roughly 2 gallons per 100 miles.

A gallon of gas releases about 20 pounds of carbon dioxide when burned.

Here's Larrick's complete post.

Here's a post that explains his formula, which is gaining acceptance in news and consumer media. Larrick is at a conference in Toronto this morning to present his mpg research and calculations.