Would cash-for-clunkers be good or bad for the environment?

Depending on the fuel efficiency of the car or truck, and how many miles it will be driven, it could take from a few years to beyond the lifetime of the vehicle to make up the “carbon cost” and begin saving on emissions.

"There are a lot of carbon emissions embedded in the production of that new car,” Dr. Chameides says. “One needs to think about that.”

The biggest savings are in getting true gas-guzzlers off the road. Replacing them with vehicles that get better gas mileage can make a huge difference in the amount of fuel burned and emissions released.

"The difference in gasoline saved between a 35-m.p.g. car and a 30-m.p.g. car is much less than for, say, a 20-m.p.g. car and a 15-m.p.g. car,” he says.

Why that is so “is not intuitive at all,” he adds. Two of his colleagues, Duke management professors Rick Larrick and Jack Soll, call it “The M.P.G. Illusion.”

For example, compare two vehicles that are both being driven 10,000 miles a year. A vehicle getting 10 miles per gallon would burn 1,000 gallons. Replacing it with a vehicle getting 20 m.p.g. would mean you’d burn only 500 gallons, a savings of 500 gallons.

But the effect diminishes in comparisons between high m.p.g. vehicles. Driving 10,000 miles in a car getting 25 m.p.g. would require 400 gallons of fuel. Switching to a miserly 50 m.p.g. vehicle (such as a Toyota Prius) would cut fuel consumption to 200 gallons. That’s a savings of only 200 gallons, despite the large spread in m.p.g.

A cash-for-clunkers plan is “very hard to justify in terms of oil-use reduction or greenhouse-gas reduction,” says Daniel Sperling, a professor of engineering and environmental science at the University California, Davis, and the founding director of the school’s Institute of Transportation Studies.

But, he says, “it possibly could make sense for conventional air-pollution reduction.”

Older vehicles emit conventional air-pollutants, such as nitrogen oxide and sulfur dioxide, at rates as much as 100 times higher than newer vehicles, he says. That’s because they have less-sophisticated pollution controls and because emission levels tend to worsen as vehicles age.

A better strategy than cash-for-clunkers, says Professor Sperling, would be to raise the CAFE standards for automakers. “That’s by far the No. 1 strategy,” he says, for cutting fuel consumption and CO2 emissions.

To ensure demand for high-m.p.g. vehicles, the government should create incentives, such as rebates for gas-sippers or raising gas taxes, he says.

Britain is considering a cash-for-clunkers plan, too. But some critics have labeled these programs as “greenwashing.” Such schemes are “nothing but hand-outs for the car firms, resprayed green to fool the incautious buyer,” writes firebrand environmental activist George Monbiott in The Gauuardian, a British newspaper.

Others wonder if replacing older vehicles with other vehicles, even if more efficient, is the most environmentally friendly way forward. The Feinstein bill does, for example, allow the cash voucher to be used to buy transit fare instead of a car or truck.

That indicates that improving the vehicle fleet doesn’t address the whole transportation picture. “I was disappointed that in the stimulus bill more money was not put into mass transit and public transit as opposed to building roads,” Duke’s Chameides says.

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