When we write about European vehicles, we usually include the standard XX grams of CO2 emitted per kilometer number that expresses how dirty a vehicle's tailpipe is. Since tailpipe emissions are (on liquid fuel vehicles) directly connected to how much fuel a vehicle burns, and it's usually a good gauge of how efficient the vehicle is. While the U.S. offers an official standardized miles per gallon number (despite some calls to shift to a gallons per mile system), there is a way to get a sort of g/km number here in the U.S.

The source is the EPA's Green Vehicle Guide, which rates vehicles based on tailpipe emissions and carbon-dioxide emissions. As Green Car Reports points out, this guide is a lot better for figuring out how clean a vehicle is than some of the media's efforts at ranking dirty vehicles (Forbes, we're all looking at you). There is some state-to-state variation, since 13 states use California's emission requirements, but at least there is a way to see which cars sold in the U.S. are literally cleaner than the rest.

Who comes out on top? The Ford Fusion Hybrid, the Honda Civic Hybrid, the Honda Insight, the Mercury Milan Hybrid, and the Toyota Prius all scored 10 out of 10 in the more stringent SmartWay Elite ranking.

[Source: EPA Green Vehicle Guide via Green Car Reports]