When it comes to pollution, these cars prove it's not just about fuel economy.
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.
"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."
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.
Even had we included those supercars, though, the Jeep Grand Cherokee still would have topped the chart. It scored a paltry three out of 10 for air-pollution ratings and two out of 10 for greenhouse gas emissions. The flex-fuel engine — prized because it uses a renewable resource that reduces dependency on traditional gasoline — on the Cherokee was even worse: three out of 10 and one out of 10 for the air pollution and gas emissions, respectively.
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In terms of finger-pointing, however, it's not just Jeep maker Chrysler that needs to clean up its act. In all, cars and trucks account for almost one-third of the total air pollution in the United States. And while EPA emission standards have gotten increasingly strict since they first were instituted in the early 1970s, there's still a long way to go, especially at home: 60 percent of the entries on our list are from domestic automakers. The remainder are German; no Japanese or Korean cars make the list.
Of American automakers, though, Chrysler is the worst offender, with six vehicles on last year's "dirtiest" list (the Jeep Commander, Jeep Grand Cherokee, Dodge Durango, Chrysler Aspen, Dodge Ram 1500 and Dodge Dakota), and five this year.
It's important to note that each of the cars on our list are street legal — and much cleaner than anything on the road years ago.
"We continue to drive our fleet average even lower," Chrysler spokesman Nick Cappa said in a written response to the rankings. "Chrysler Group products are 99 percent cleaner than vehicles of 30 years ago and meet or exceed United States federal emission standards, the most stringent in the world."
Many auto manufacturers are making concerted efforts to produce models that are easier on the environment; most offer hybrid, compact or turbocharged 4-cylinder versions, all of which record low pollutant scores. Next year will see the long-awaited arrival of several low- and emission-free options, like the compact but "ecoboosted" Ford Fiesta and the plug-in electric Chevrolet Volt.
Some high-end carmakers are making considerable emissions efforts as well, but don't look to them for the latest in high-volume electric technology. It's prohibitively expensive and technologically difficult to get anywhere near the same performance out of an electric motor as a combustion engine, and aside from notable entries from Tesla and (perhaps) Fisker, it will be years before any meaningful amount of electric motors find their way into high-performance and luxury cars.
Besides, there's much to be done to improve the combustion engine, engineers from Bentley, BMW and Porsche say, either through increasing its efficiency or developing alternative fuels. Bentley developed the flex-fuel-capable, $245,000 Continental Supersports coupe for that very reason.
"We believe bioethanol is a really good alternative to reducing CO2 emissions," says Brian Gush, Bentley's director of chassis and powertrain engineering. "This is a renewable source, which will be growing into the future."
Simon at the EPA says emissions levels have indeed improved significantly over the last 20 years and will continue to do so. But with 40,000 Jeep Grand Cherokees sold this year so far, there's a long way to go.
© 2012 Forbes.com