The greenest car you've likely never heard of will soon be hitting Honda showrooms across the United States as the Japanese automaker expands sales of its compressed natural gas powered Civic.

Honda has been quietly winning green car awards for more than a decade as it cautiously introduced the Civic GX first to government and business fleet owners and then retail customers in a handful of test markets.

The nationwide retail launch set for this fall comes as US President Barack Obama pushes for wider adoption of fuel-efficient vehicles -- including mandating that all federal cars will need to run on alternative, hybrid or electric power by 2015.

Potential customers could also be lured by substantial cost savings as oil prices climb amid tensions in the Middle East and natural gas prices fall in the wake of major new discoveries in the United States.

But the Civic GX enters a crowded field where new plug-in hybrid and fully electric cars -- the Chevy Volt and Nissan Leaf -- are grabbing headlines and zippy new compact cars offer competitive fuel economy.

Honda's goals are relatively modest -- doubling sales to around 4,000 vehicles in the first year of national sales while Nissan is hoping to hit annual US sales of 20,000 Leafs -- but it still thinks the GX can compete.

"We're asking the GX purchaser to make far fewer sacrifices than any other alternative fuel vehicle," Eric Rosenberg, who heads Honda's alternative fuel vehicle program in the United States.

"When you compare it to the Volt or Leaf, it's the most affordable, it has the best range and it has the quickest refill."

The GX can drive up to 250 miles (403 kilometers) on a single tank and only takes a few minutes to fill at public or home fueling station.

The Leaf has a range of 62 to 138 miles (100 to 222 kilometers) depending on road conditions and takes 30 minutes to partially charge at a quick-charge station and seven to 20 hours using a standard 220 or 110 volt outlet.

GM's Volt can drive 25 to 50 miles (40 to 80 kilometers) on its battery before switching over to a gasoline-powered engine and takes four to ten hours to charge.

Honda's GX is also the cleanest car on the US market, according to the American Council for an Energy-Efficient Economy which looks at a vehicle's total environmental impact.

That's because natural gas is a clean-burning fuel. It consists primarily of methane and emits about 30 percent less carbon dioxide and 70-90 percent less smog-forming particulates than gasoline.

Electric cars may emit nothing from the tailpipe, but they have a significant carbon footprint because 45 percent of US electricity is generated by coal. Their batteries also carry a heavy environmental toll.

Realtor and property manager Irma Vargas bought her first Civic GX in 2006 to save on fuel costs and get access to carpool lanes -- a perk that can cut a 90-minute commute in half in congested Los Angeles.

"Me and my business partner bought it and were going to take turns with it because it was a new idea," Vargas said in a telephone interview.

"We found that we were fighting over it, so he ended up getting the next year's model."

Vargas sold the GX to an employee so she could upgrade to a new model in 2008 and has convinced four of her friends and customers to buy one as well.

She figures she's saved thousands of dollars on fuel costs -- she can fill her GX at home for about a dollar a gallon while it costs nearly four dollars a gallon to fill her Lexus hybrid, which she saves for long trips and big shopping excursions.

But it will be years before the GX or electric cars are sold in sufficient numbers to make a significant dent in greenhouse gas emissions, cautioned Lonnie Miller, an analyst at auto research firm R. L. Polk.

"If you look at the traditional batch of gas-electric hybrids, it's 2.6 percent of all US new vehicle registrations," he told AFP.

"CNG (compressed natural gas) and electric, they're not even registering."

It took six years for US consumers to embrace hybrids, which require only a few tradeoffs like a higher initial price tag and limited trunk space.

Like fully-electric cars, the Civic GX requires a much bigger tradeoff.

While owners can fuel up at home with relatively cheap unit called "Phil," long-range trips are essentially out of the question because there are only about 870 public fueling stations in the entire country.

The cost and environmental advantages of compressed natural gas will nonetheless help boost global sales by 9.1 percent a year to 3.2 million vehicles in 2016, according to a recent report by green tech consulting firm Pike Research.

The biggest growth -- 25 percent a year -- is forecast in the United States, fueled primarily by sales to corporate and government fleets which typically operate their own fueling stations.

Honda started with fleet sales in 1998 and offered the GX to retail customers in California and New York in 2005 as more fueling stations came online.

It expanded retail sales to Utah and Oklahoma in 2008 and 2009 as tax incentives in those natural-gas producing states drew more customers, but has only sold a little over 12,000 of the vehicles so far.

"The whole idea was for us as a company to learn how to retail the car," Honda's Rosenberg said. "It is a little different, it has a few idiosyncrasies."

The experience with the relatively-low cost GX will serve as a good model for the introduction of the holy grail of green cars -- the far more expensive and complex hydrogen fuel cell.

Honda was the first automaker to introduce a hydrogen fuel cell prototype in 1999 and began testing a small fleet with retail customers in 2005. It also plans to introduce a plug-in hybrid next year.