America's dirtiest vehicles

Just because a car has low fuel efficiency doesn't mean it's the worst polluter on the road. The Chevrolet Suburban and Dodge Challenger are some of the biggest gas-gulpers available, but they don't cause quite the environmental harm other cars do.

That title is reserved for vehicles that combine their poor gas mileage with high tailpipe and greenhouse gas emissions. Think along the lines of some of the bulkiest cars on the road, like the Jeep Grand Cherokee, Chevrolet Trailblazer and Dodge Dakota.

While fuel economy is linked to emissions, it's not the only factor. Pollution levels also have to do with the type of fuel being used and the age and condition of the engine, among other things.

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"You can have a really fuel-efficient, dirty vehicle and a really clean, not-so-fuel-efficient vehicle," says Karl Simon, a director of compliance and innovation for the EPA's Office of Transportation and Air Quality. "It really is pretty wide open from a technical perspective."

Behind the Numbers

To determine the dirtiest cars on America's roads, we used air pollution and greenhouse gas emissions data from the U.S. Environmental Protection Agency.

The EPA rates air pollution on a scale of 0 to 10; the score reflects the amount of tailpipe emissions a vehicle releases. Vehicles that score 10 are the cleanest--they don't emit pollutants like hydrocarbon, nitrous oxides, particulate matter, carbon monoxide and formaldehyde. Greenhouse gas levels (carbon dioxide, nitrous oxide and methane) are based on the vehicle's fuel economy and are evaluated on the same 0-10 scale. That score represents the "relative global warming potential of each car," the EPA says.

For our list, we combined air pollution and emissions scores and then chose the vehicles with the smallest results (the greater the score, the more environmentally friendly the car). We broke ties by evaluating the combined fuel efficiency of each vehicle. (Driving a car that gets 25 miles per gallon rather than 20 mpg will prevent 10 tons of carbon dioxide from hitting the air over a vehicle's lifetime, according to EPA data.) We did not evaluate vehicles classified as "heavy duty," like the 3500 series of the Dodge Ram, which are exempt from federal fuel economy requirements.

We also deliberately omitted some vehicles that rated higher on the particulate-emissions scale, including exotics like the Ferrari F430, uber-luxury cars like the Bentley Continental Flying Spur, and high-performance variants from Mercedes-Benz's AMG line. Those cars have marginally worse emissions ratings than some of the entries on our list but are produced in such small quantities and driven so infrequently--on weekends or on racetracks--that they often don't contribute much to air-pollution problems.

Lonnie Miller, an automotive analyst for R.L. Polk, says it's OK that performance and design dictate the aim of those vehicles, while others emphasize fuel economy and practicality. There's a place in the market for each type, he says.

Image: Dodge Durango