By Ros Krasny

WASHINGTON (Reuters) - General Motors Co will begin selling a mid-sized sedan next summer that can be powered by either gasoline or compressed natural gas, the U.S. automaker's chief executive said on Wednesday.

The 2015 Chevrolet Impala, GM's first car powered by natural gas, will feature a powertrain that switches from compressed natural gas to gasoline seamlessly and has a total driving range of up to 500 miles, Dan Akerson said in a speech to be delivered at an energy summit in Washington.

The car, which will have one fuel tank for compressed natural gas and a second one for gasoline, will be sold to both retail and fleet customers.

Natural gas is a cleaner-burning, less costly fuel than gasoline, and vehicles powered by compressed natural gas typically emit 20 percent less greenhouse gases than gas-powered cars, GM said, citing the California Air Resources Board.

New techniques unlocking vast reserves of natural gas from shale have produced a boom in U.S. supplies and driven down prices, increasing interest in the fuel.

The numbers of CNG vehicles remain small. According to the industry group Natural Gas Vehicles for America, about 130,000 to 135,000 natural gas vehicles operate in the United States and more than 16 million globally, most of them commercial and fleet vehicles such as buses and garbage trucks.

The number of natural gas filling stations totals about 1,350 in the United States, about half of which are open to the public. That compares with about 168,000 retail gasoline stations, Akerson said.

GM previously said that next year it would begin selling bi-fuel versions of its heavy-duty Chevy Silverado and GMC Sierra pickup trucks and natural gas versions of its Chevy Express and GMC Savana passenger vans.

Honda Motor Co sells a CNG-powered Civic. Ford Motor Co prepares its trucks and vans so that specialty companies can convert them to run on compressed natural gas, including its top-selling F-Series pickup truck, starting next month.

The bi-fuel Impala is meant to address range anxiety associated with vehicles that operate on natural gas only, much like GM's Chevy Volt plug-in hybrid car was designed to do for electric cars, Akerson said. It will carry a factory warranty on the powertrain and fuel system.

GM did not disclose how much the bi-fuel Impala will cost, but regular versions start at about $27,700.

"Natural gas powertrains are one of the areas where we have increased investment because we believe the technology can satisfy the 'green' needs of both the environment and the stockholders," Akerson said in the speech.

Citing the lack of CNG gas stations, Akerson said the volumes for the bi-fuel Impala will initially be small with most sales to commercial and government fleets. He said selling 750 to 1,000 of the cars in the first model year would be "a home run."

He also repeated his call for the Obama administration and Congress to create a consumer-driven national energy policy. In March, he said President Barack Obama should appoint a commission to develop a 30-year U.S. energy policy framework that includes energy producers, labor groups and energy consumers such as GM.

In March, Akerson also said that natural gas as a motor fuel represents a "huge and largely untapped opportunity for commercial fleets and long-haul truckers."

(Additional reporting by Ben Klayman in Detroit; Editing by Steve Orlofsky)