The 179,000 garbage trucks in the United States consume 24 million barrels of diesel fuel annually, according to INFORM, a New York-based environmental group.

Trash trucks are the least fuel-efficient vehicles on the road, averaging 2.8 miles per gallon, the group said in a report.

The report, called Greening Garbage Trucks, surveyed 25 U.S. cities including New York, Los Angeles and Washington, D.C.

It found that 90 percent of garbage trucks are powered by diesel fuel, but that the rest of the vehicles have converted to cleaner natural gas and liquefied natural gas (LNG).

The gas-fueled trucks emit 70 percent to 90 percent less particulate matter than diesel, 30 percent to 70 percent less of acid rain component nitrogen oxide, and 10 percent less carbon dioxide, a greenhouse gas thought by scientists to cause global warming by trapping the sun's heat in the Earth's atmosphere.

In California the South Coast Air Management District mandates that townships replace diesel trucks with natural gas and NGL trucks as the conventional trucks retire.

David Rodriguez, the fleet manager for the city of Santa Monica, California, said nearly all of the city's 22 garbage trucks have already been converted to run on natural gas and are equipped with John Deere "81" engines. "They are holding up very very well," said Rodriguez when asked how the trucks perform versus conventional engines.

FROM ROTTING GARBAGE TO FUELING TRUCKS

The savings in greenhouse gas reductions of gas-fueled trucks has also helped Waste Management Inc. , the leading U.S. garbage hauler, garner greenhouse gas reduction credits. In turn, that helps it buy more green trucks.

Under the California mandate, utility PG&E Corp. has helped Waste Management pay for the conversion of trucks to run on natural gas and LNG.

Waste Management's "supply" of emissions reductions allowed it to sell credits to a companies that needed to buy the right to emit greenhouse gases. PG&E bought the credits to offset emissions of a power plant the company was building in San Diego county.

Waste Management also siphons off potent greenhouse gas methane formed from rotting garbage at its landfills, which also helps the company get emissions credits.