The Associated Press

But they're having a harder time filling up the tank.

Some drivers say they're running into problems with pressure when they try to pump the fuel into the specialized vehicles.

They say the stations that offer natural gas aren't operating at full capacity and some pumps are running on empty.

On Monday Lehi's Brady Knight said only two of eight pumps were working properly at a Questar Gas Co. fueling station in Salt Lake City.

"We've spent the last hour trying to find a natural-gas station," said Knight, who spent just $6.46 for 7.52 gallons of compressed natural gas.

Questar spokesman Chad Jones said the pressure runs low when drivers fill up one after the other and the pumps don't have time to recover.

The tanks are like large balloons that lose pressure when fuel is pumped out and need time for the compressor to refill, Jones said.

The demand for natural gas for vehicles has increased more than 220 percent in the last year, Jones said.

Technology upgrades and improvements to the natural gas delivery systems could alleviate some of the problem, Questar analyst Darren Shepherd said.

But upgrades at stations could cost hundreds of thousands of dollars per location and would take either investment from private natural-gas fuel providers, or a significant increase in the supply of natural-gas vehicles, he said.

"We are getting some calls from retail stations, and we've got a team of folks taking a look at our infrastructure and other options," Shepherd said. "We are hoping that we see the private sector step it up a little bit."

Despite the pressure problems, drivers of natural gas-powered vehicles are paying just about 86 cents per gallon compared to about $4 per gallon of gasoline.