Ask Mr. Green: What's the Deal with Smog Scores on Cars' Window Stickers?

Hey Mr. Green,

I'm new-car shopping and am confused by the global-warming and smog scores posted on cars' window stickers. I was surprised to see a small, fairly fuel-efficient Fiat 500 get a score of 4, which seems low. What gives?

--Glenn in Chapel Hill, North Carolina

What gives is a complicated system of rating vehicles according to three key factors: miles per gallon, emissions of global-warming gases, and emissions of dangerous chemicals and lung-lacerating microparticles.

So while vehicle A might get the same gas mileage as vehicle B, it could emit more pollutants. In other words, because of different engineering designs, there isn't always a direct correlation between the amount of fuel burned and the amount of crap spewed into the air.

Consider this example: Traveling 15,000 miles, the Chevy Aveo emits half a ton more CO₂ than the Ford Fiesta, yet the two models have the same air-pollution score. At the EPA's fueleconomy.gov site, you can compare the miles per gallon, CO₂ emissions, and air score of cars dating back to 1984.

A caveat: The EPA's rating of electric cars is conceptually flawed, in that it compares electrical energy with gasoline but doesn't include the energy needed to generate the electricity; so it understates electric cars' total energy use and CO₂ emissions. That said, electric cars are still much cheaper to operate, because gas is more expensive than the fuels used to make electricity, and because gasoline engines are far less efficient than electric motors.

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