Forbes Autos posted a detailed roundup of cars that pollute the atmosphere the least. The report focuses on gas and hybrid cars, and leaves out potential clean diesel advances—which isn't entirely unreasonable, since the first clean diesels won't even be available here until later this year. Still, there are a number of surprises in the story.

"Internal combustion engines emit a host of harmful gases as a result of burning fossil fuels like gasoline, diesel, and ethanol," the report said. "These highly toxic smog-forming emissions include carbon monoxide, formaldehyde, and other particulate matter that create the dark smoke seen billowing from diesel trucks and the gray haze hanging over large cities.

"In addition to those pollutants, which are harmful to human health, vehicles also emit what the U.S. Environmental Protection Agency calls 'greenhouse gases,' with carbon dioxide (CO2) being the main offender. Motor vehicles are the largest source of CO2 emissions, accounting for 51 percent of total emissions, according to the EPA. Burning one gallon of gasoline emits 20 pounds of CO2."

The rankings are very specific, and come down to exact models with specific engines and transmissions. Cars in the slideshow include the Infiniti G37 coupe with a stick, the rear-drive Mercedes C300 with a stick, and several Lexus hybrids. Check out the full slideshow here.

The story noted, for example, that while the BMW 328i with a stick makes the list, the 328xi doesn't: The former outputs 8.3 tons of greenhouse gases per year, while the latter outputs 9.2 tons, even though they're the same car with the same engine and transmission, because the xi version achieves poorer fuel economy numbers.