DETROIT -- Ford Motor Co. says it sold a record 11,600 natural gas vehicles last year, more than four times the number it sold two years ago.It's the latest sign that natural gas is making inroads as a transportation fuel, particularly for truck fleets, buses and taxis. The consumer market is tougher to crack, but sales are gaining there as well.Natural gas is cheap and plentiful in the U.S. after a spike in production that began in the middle of last decade. At the same time, the price of gasoline and diesel fuel has jumped more than 30 percent.That makes natural gas -- which also emits fewer greenhouse gases -- an increasingly attractive option for truck companies and municipalities.Irving-based oil and gas company Pioneer Natural Resources recently ordered 250 F-250 pickups that can run on a combination of gas and natural gas to add to its 50 bi-fuel vehicles, says Lynn Lyon, Pioneer's director of fuel market development."People will drive across town to save 10 cents a gallon at the pump. What would you do to save $1 or more?" she said.But while natural gas may be a good choice for snow plows and trash trucks, which go relatively short distances and can refuel at city-owned pumps, it's a tougher call for ordinary consumers. Natural gas cars cost more and there are few public places to refuel them.With a starting tag at $26,305, a 2013 natural gas Civic costs $8,100 more than the base gas model. While big trucks burning 20,000 to 40,000 gallons of gas a year can make up that difference, it's harder for regular motorists, who may only use 500 gallons annually.Those issues need to be addressed if the vehicles are to significantly boost their share of the auto market, which is currently less than 1 percent.General Motors and Chrysler recently added natural gas pickup trucks to their lineups. Honda is seeing more interest in its natural gas Civic -- with record U.S. sales of nearly 2,000 last year -- and industry experts expect more offerings for regular buyers in the next year or two.In Houston, General Motors CEO John Akerson told a gathering at the CERA energy conference that the automaker by year's end will offer buyers of its heavy-duty pickup trucks the option of an engine that can burn both gasoline and also compressed natural gas, or CNG. He said GM also plans vans and some trucks, aimed at fleet operators, that burn CNG.Not in ArlingtonBut those engines aren't expected to find their way into the company's full-size sport utility vehicles built in Arlington. GM has expanded the Arlington assembly plant, adding a shift and a stamping plant to the company's only facility producing the Chevrolet Tahoe, GMC Yukon and Cadillac Escalade.Instead, full-size SUVs for 2014 will use engines similar to those planned for large pickups, which use advanced technology like turbocharging, direct injection, cylinder deactivation and variable valve timing to boost efficiency, Akerson said.Natural gas vehicles aren't new. Ford's previous peak sales, of 5,491, were in 2001. But they fell out of favor later that decade when the price of natural gas spiked. Ford stopped selling natural gas vehicles in 2004 and didn't start making them again until 2009.During those five years, new technology unlocked vast reserves of natural gas in deep rock formations, creating a glut that has depressed prices. Compressed natural gas -- or CNG -- now costs between $1.79 to $3.49 per gallon in the U.S. depending on the location, compared with an average of $3.74 for gasoline and $4.12 for diesel, according to Clean Energy, which operates natural gas fueling stations, and AAA.No one is quite sure how many natural gas vehicles are on the road. Honda and Chrysler are the only companies that make CNG-ready vehicles in their own factories. Ford and GM make vans and trucks that are prepped to run on CNG, or on a combination of gasoline and CNG, but rely on outside companies to add about $10,000 worth of equipment, including the natural gas tank. Some drivers convert their cars and trucks on their own.GE estimates there are 250,000 natural gas vehicles currently in use in the U.S.Analyst Dave Hurst of Pike Research estimates that 20,381 natural gas vehicles were sold in the U.S. in 2012 -- more than half by Ford but big truck makers like Navistar and Freightliner were also in the mix.Staff writer Jim Fuquay contributed to this article.