-LRB- AOL Autos -RRB- -- Regular readers of AOL Autos know that we have done a series of stories on the development and increasing popularity of cars that run -- or will eventually run -- on alternative fuels . The mandatory use of CNG in public service vehicles began in New Delhi , India in 2000 . We 've written about hybrids , clean-diesel engines , fuel-cell technology , ethanol and more . That brings us to another entry in the auto industry 's ongoing research and development of green-technology : compressed natural gas -LRB- CNG -RRB- . Scientists are trying to determine which alternative fuel will best strike a balance between being environmentally friendly and commercial viability . CNG might be the answer . Vehicles running on CNG have actually been around since the early 1990s , but have not been a dominant force in the marketplace mostly because the infrastructure -LRB- i.e. re-fueling stations -RRB- is not yet in place to support high-volume sales of CNG-powered cars . Presently , there are only about 1,600 CNG refueling stations in the U.S , compared with up to 200,000 gas stations . Currently there is only one CNG-powered model sold on the consumer market that is actually manufactured as a CNG-powered car . That 's the Honda Civic GX , which boasts an EPA-estimated fuel economy of 24/36 miles per gasoline-gallon equivalent . AOL Autos : Best hybrids Rich Kolodziej , president of Natural Gas Vehicles for America said there are other natural-gas-powered vehicles on the road -- about 130,000 , he estimates -- but that the vast majority of those are part of commercial or transit fleets or have been converted from gasoline-powered cars . `` They 're mostly fleet vehicles , like transit buses , delivery trucks , and the fleets maintained by the gas company , the electric company , etc , '' he said . `` We do n't have hard figures about conversions , but I 'd bet that about 25 percent of those 130,000 were once gasoline-powered vehicles that have been converted to CNG vehicles , '' Kolodziej said . `` There are a growing number of companies making certified conversion systems and installing them . '' NGVAmerica maintains a list of certified conversion systems on its Web site : www.ngvc.org . AOL Autos : Fuel-efficient used cars Some of those 130,000 are also natural-gas-powered vehicles manufactured and sold by Ford , Chrysler and GM back in the early '90s , he says -- back when the U.S. manufacturers were still in the business of making CNG-powered cars . But we 'll get back to that later . But with the price of gasoline currently averaging over $ 4 a gallon nationally , Kolodziej predicts there will be more and more demand for CNG-powered vehicles like the Civic GX . `` I went to a conference a couple of months ago , and every manufacturer we talked to said that their phones were ringing off the hook , from people who are saying they 'd be interested in buying a CNG-powered car , '' Kolodziej said . `` And there are more and more people and companies who want to get into the conversion business . '' AOL Autos : Hybrid SUVs The cost of conversions varies by the type of vehicle , Kolodziej noted . `` But most cost in the $ 10,000 range . Most conversion systems are certified for Ford and GM models , among others , '' he said . On the financial side , natural gas is about 30 percent less expensive than gasoline when it is purchased at a refueling station . It 's about 50 percent less expensive when you fill ` er up at home , via home refueling appliance that tap into your natural gas line , Kolodziej said . Owners of the Honda Civic GX in New York and California who have natural gas piped into their homes can purchase the `` Phill '' re-fueling system for about $ 3,500 . This Phill system can be used whether your CNG vehicle was manufactured that way or converted , he said . AOL Autos : Luxury cars with best gas mileage `` The Phill is about the size of a pay phone , and you hang it on the garage wall , and vent it like a dryer and plug it into a gas line and also into an electric socket , '' Kolodziej explained . The primary environmental benefit of a CNG car is that it produces 90 percent fewer smog-forming pollutants -LRB- oxides of nitrogen -RRB- , compared to the output of a conventional gas engine , said Steve Ellis , manager of alternative fuels for American Honda Motor Co. . The Civic GX has reaped many environmental awards . This year , it was named by the American Council for an Energy-Efficient Economy -LRB- ACEEE -RRB- as the `` greenest vehicle in the nation -- for the fifth consecutive year . '' The GX , which was first introduced in 1998 , has the cleanest internal combustion certified by the EPA , Ellis noted . AOL Autos : Most-popular fuel-efficient cars The Civic GX is not a high-volume vehicle in terms of sales . Ellis said annual sales have fluctuated between 500 to 1000 units a year since it was rolled out , with sales of 1,100 units last year . He expects that to go up in 2008 . For comparison purposes , about 40,000 Honda Civic Hybrids were sold last year , with sales of 48,000 anticipated for this year . Most of the sales of the Civic GX are concentrated in southern California , followed by Phoenix and Salt Lake City -- primarily because those markets are where many of the 1,600 re-fueling stations are presently located . `` As the infrastructure builds up , we 'll sell more of them in other markets , '' Ellis predicted . `` Right now , this is a car that people use regionally . If you live here in Los Angeles , you can drive to Las Vegas and back and not worry about being able to find a refueling station . As for cost , the GX natural-gas vehicle is priced at about $ 24,500 , compared to $ 17,000 for a comparably-equipped Civic LX , Ellis said -- except that buyers get a $ 4,000 tax credit on the purchase of the GX . And until recently , California buyers got an additional $ 3,000 rebate from the California Air Resources Board . `` But the program was so popular , they are out of funds , and looking to add more money to the coffer so they can continue with the rebates , '' Ellis said . `` But people who buy a Civic GX , when we ask them about the math , in term of comparing those two Civics , they sort of give us blank stares , '' Ellis continued . `` Most of these folks who are buying the GX are trying to get out of having to commute to work every day in a pick-up truck , SUV or even a passenger car that 's only getting 15 or 20 miles a gallon . And they 're also the same kind of people who 've said they 'd pay $ 25 or $ 50 a week to be able to drive in the car-pool lane . '' Interestingly , many of the automakers , like Volkswagen , Mercedes , Fiat , GM Opel , Ford Europe , Peugeot and Renault , do produce CNG-powered vehicles for other countries , said NGVAmerica 's Kolodziej , who noted that `` about 1.5 million of the natural gas vehicles have been sold in Brazil , another 1.5 million in Argentina , another 1.5 million in Pakistan , and so on . '' And , once upon a time , in the early '90s , U.S. automakers did sell CNG vehicles in the United States . `` But the federal government kept adding more loopholes to the energy policy that was supposed to encourage the increased manufacture and sales of CNG vehicles , '' Kolodziej said . `` Then the government opted not to expand the coverage of the program the way the law allowed -- or the way the automakers expected . Without that demand pull , the U.S. automakers did n't believe they could sell enough of them here . '' `` U.S. automakers zigged when they probably should have zagged '' added Ellis of American Honda . `` They turned their backs on CNG and began to focus on bio-fuels . '' But , noted Ellis , `` We 're definitely keeping our stake in the CNG business , because we know we can add it to other platforms . We used to think of the Civic GX as ` The Little Engine That Could ' -- you know , saying , ' I think I can , I think I can ' in terms of becoming more commercially viable . `` But , with gas prices going up and up , now we 're thinking more along the lines of ' I know I can , I know I can . ' And more and more people are genuinely interested in going more green because of the environmental impact , '' he said . `` A lot of people got into the hybrids , and then , after a while , began to ask , ` How can I do more ? ' And they 're looking to natural-gas-powered cars as the answer to that . `` So , we 're in this for the long haul , '' noted Ellis , who said the company is definitely looking to do bigger business and make a profit from the GX . `` Now , it 's just a matter of society continuing to change , and to continue to become more environmentally-minded . As they do , we think there will be increasing interest in CNG , and we 'll see a shift in emphasis toward CNG and away from bio-fuels . ''