Gas-sippers such as the Geo Metro produce only one-fifth as much carbon dioxide as some cars manufactured by luxury automakers such as Rolls-Royce and Aston Martin, a consumer advocacy group says.

Public Citizen, founded by Ralph Nader, on Tuesday released rankings of more than 900 cars, vans and light trucks from the current model year for fuel efficiency and carbon dioxide output.Carbon dioxide, a product of burning fossil fuels, is the chief culprit in what many scientists believe is the warming of the globe. A gallon of gasoline generates almost 20 pounds of carbon dioxide when burned.

"Average fuel efficiency of new cars on the road could have been 33.4 miles per gallon in 1991 - nearly 20 percent higher than the average of 28.2 mpg for all 1991 cars - if consumers had chosen only the most efficient models in each size class," Public Citizen said.

Most cars on the group's list of the 30 biggest air polluters were described as expensive specialty cars or luxury cars.

Robert Lockhart, a Public Citizen energy analyst and the report's primary author, acknowledged that larger cars are popular. But he said automakers could boost sales of low-pollution cars by offering gas-sipper rebates. Congress should boost taxes on gas guzzlers, he said.