By John Carey

On July 27 the federal government launched a cash-for-clunkers program. Trade in your old gas guzzler for a higher-mileage new car, and Uncle Sam will give you up to $4,500.

The program has been a huge success—at least in terms of giving a boost to automakers. The response from consumers has been so swift that the initial $1 billion allocated for the program—scheduled to run into November—has nearly run out. Economists say the money is giving the economy a quick stimulus.

But what is the program doing for the environment, or to reduce American's gasoline consumption?

Let's look at some simple numbers:

For that $1 billion, Americans will trade in roughly 250,000 cars and light trucks. The average gas...