Tailpipe Test: Study Finds Worst Polluters

The make of your car makes a big difference in how much it pollutes, according to a comprehensive comparison of pollution levels by manufacturer.

Age is a huge factor, too. A vehicle from 1985 emits almost 38 times more carbon monoxide than a 2001 model, on average.

Researchers analyzed data from nearly 4 million cars and trucks that underwent government-mandated emissions testing in Illinois, Maryland, and Massachusetts.

The best and worst

BMWs did best. Hondas and Volvos also tended to pass exams more often than other makes, but were inconsistent from state to state.

Hyundai, Mitsubishi, Chrysler, and GM cars were the most likely to fail.

For trucks, Honda, Toyota, and Nissan performed best. Ford was a distant fourth. Chrysler and Mitsubishi failed most often.

"There are several factors that determine how much cars and trucks pollute, and vehicle make is one of the more significant," said study co-author Jean-Michel Guldmann of Ohio State University. "The effects were uniform, but the magnitudes were different depending on the make of the car. Some makes are cleaner than others."

Other findings

Researchers also noticed the following trends:

  • Poorly maintained vehicles were more likely to fail.
  • Increased fuel efficiency in both cars and trucks resulted in decreased failure rates.
  • Increases in miles driven resulted in increased failure rates.
  • The older the vehicle, the higher the likelihood of test failure due to increased engine and emissions equipment deterioration.
  • Automobiles emit fewer pollutants on average in spring and summer, probably because of different atmospheric conditions and changes in fuel mixtures produced by refineries.

Foreign cars are not necessarily better, Guldmann said. Toyotas have the least difference between older and newer cars, because their older cars already polluted less. Other foreign makes—Honda, Nissan, and Hyundai—got much worse with age, with older models showing high emissions levels.

The finding that cars with better gas mileage also polluted less is important, Guldmann said. It suggests that current regulations, which measure emissions in grams per mile driven, should be changed to grams per gallon of gasoline used.

"This would ensure that higher fuel economy standards are automatically translated into emissions reductions," Guldmann said.

The results are detailed in the January issue of the journal Transportation Research Part D.