You may not need to pay extra for a hybrid. Here are seven economical gas-engine choices. By Mark Solheim, Senior Editor September 30, 2007 Want a quick and not-so-dirty alternative to an alternative-energy vehicle? Buy a super-fuel-efficient car. Every gallon of gas burned releases 20 pounds of carbon dioxide, the primary greenhouse gas, into the atmosphere. The less gas you burn, the more you reduce your contribution to global warming. RELATED LINKS Best Values in Clean Cars SLIDE SHOW: Earth-Friendly Autos Scorecard Tax Breaks for Green Cars According to the American Council for an Energy-Efficient Economy, a bunch of four-cylinder-engine cars -- including the Ford Focus, Honda Fit, Kia Rio, Mazda3, Nissan Versa and Toyota Corolla -- earn better "green scores" than all but four of the hybrids on the market. Green scores are based on fuel economy as well as tailpipe pollutants. The ultimate gas sipper, at about 40 miles per gallon, is the Smart ForTwo (www.smartusa.com). The three-cylinder-engine Daimler microcar, a common sight in Europe, arrives in the U.S. in early 2008. It is only 106 inches long -- half the length of a Ford Crown Victoria -- and is expected to start at less than $12,000. NEXT: See the "green scores" for 14 cars in our Earth-Friendly Autos Scorecard slide show.