So, what did we get for our $3 billion?

The government's Cash for Clunkers trade-in program, which offered Americans up to $4,500 toward the purchase of a new, more fuel-efficient car when they traded in an older car, burned through $3 billion in just 29 days. Advocates had promised big things from the initiative - environmental benefits as the program pulled over-polluting older cars off the road, economic benefits as it poured rocket fuel into a depressed auto market, even energy and foreign policy benefits as it cut our demand for oil.

Government officials have already labeled the Car Allowance Rebate System (CARS) a resounding success. Transportation Secretary Ray LaHood, announcing plans to wind down the rebate program, called it "the economy's best story in six months."

Others, however, aren't so sure. Let's look at the numbers.

The Department of Transportation hasn't announced any official tally of CARS-related auto sales. It may not be able to for some time - administrators face a massive backlog of applications for vouchers. But DOT officials estimate that around 750,000 CARS transactions will have been processed by the time the program's funding is exhausted.

The government, however, did provide a mid-program update on August 5. At that point, the average clunker traded in had a fuel economy rating of 16 mpg. In order to qualify for the full $4,500 voucher, a consumer had to buy a new car with an EPA rating of 10 mpg more than their trade-in. Let's assume that every one of those 750,000 transactions saw someone boost their gas mileage by 10 mpg - from 16 mpg to 26.

In truth, we know the results will be less impressive than that - DOT figures show that nearly half of buyers have seen less than a 10 mpg improvement. But we want to know what the best-case scenario might be.

Energy Benefits

According to EPA estimates, the average American drives 15,000 miles every year. By that measure, at 16 mpg, each clunker was consuming about 937 gallons of gas per year. Its more efficient replacement would consume about 577 gallons - an improvement of 360 gallons per year. Seven-hundred and fifty thousand such improvements would cut nationwide consumption of gasoline by 270 million gallons per year.

It sounds impressive - until you realize that Americans consume 378 million gallons of gasoline each day. In an unrealistically rosy scenario, then, Cash for Clunkers may have cut our oil consumption by less than 0.2 percent per year. It didn't even save us a day's worth of gas.

Environmental Benefits

Perhaps, then, the program's real benefits came in the form of emissions cuts.

According to the carbon offset firm Terrapass, each individual driver putting 15,000 miles on their 16-mpg car would produce 18,341 pounds of CO2 annually. Driving a 26-mpg car instead, they would produce 11,287 pounds each - an improvement of 7,054 pounds. Seven-hundred and fifty thousand such improvements would mean a 5.3 billion-pound reduction in our nationwide carbon output. Again, that sounds impressive - until it's put in perspective. Our total nationwide carbon footprint is hard to estimate, but Department of Energy projections put it somewhere north of 540 billion pounds annually - meaning that, even in our unrealistically positive scenario, Cash for Clunkers cut it by less than one percent.

And, critics point out, this calculation doesn't include the carbon emissions involved in building new cars, shipping them, and scrapping old cars.

Economic Benefits

Okay, if it didn't cut our gas consumption or our carbon emissions much, did the program at least have an economic benefit?

IHS Global Insight, an automotive research firm, is one of several industry analysts to revise its expectations in light of the program's success. A month ago, IHS projected nationwide sales of 9.8 million cars in 2009. This week, they're projecting 10.3 million.

But again, a little perspective is telling. As recently as 2007, Americans bought nearly 17 million cars. Cash for Clunkers caused a sales boost - but it still didn't bring the automotive market much past half the size it was just two years ago.

What's more, some analysts wonder if Cash for Clunkers really created any new sales at all. It's possible that the program simply caused some people to buy cars earlier than they otherwise would have. What good comes from adding to August sales if we subtract from future sales to do it?

Even if it brought in new buyers, that scenario might create its own economic problems. The program led hundreds of thousands of Americans, for instance, to take on new debt in the midst of a recession and an uncertain job market.

Room For Optimism

It's not all bad news, however. Even if the benefit of Cash for Clunkers was purely psychological, that might be worth the money.

Some economists say that a phenomenon called the "paradox of thrift" is feeding the current recession - Americans are scared for our jobs, so we aren't spending. Since we're not spending, the economy can't grow. If Cash for Clunkers created anything, it was a round of positive economic news that may help convince Americans that it's safe to spend again.

In a blog post touting the program, LaHood argues that Cash for Clunkers has already led automakers to pay workers money they otherwise wouldn't have earned. "Both Ford and GM have announced they are stepping up production to meet the new demand this Administration has stimulated," he explains. "GM is calling back over 1300 employees. Can you imagine? In this economy, an automobile manufacturer is calling back its workers." Increased production will have downstream benefits as well, as the companies pay suppliers and re-employed workers pay bills.

What Did We Get for Our $3 Billon?

Whether or not all the incremental benefits are worth $3 billion is an open question. The inherent problem in unpacking the costs and benefits of such a program is that the costs are all up front, and obvious, but the benefits are spread out and harder to measure. If the spending spree helped to trigger an economic recovery, perhaps it was worth it. If not, perhaps there are more efficient ways to buy environmental and economic stimulus.