Why would a company buy thousands of trucks and vans fresh off the assembly line and install a bulky and expensive new fuel system? It comes down to the bottom line.

In a time when natural gas is relatively cheap, but manufacturers build mostly gasoline and diesel models, companies like AT&T and Verizon have calculated it's worth the expense to convert some gasoline vehicles to burn compressed natural gas (CNG).

Worldwide, natural gas vehicles make up a tiny sliver of the auto pie, accounting for just under one percent of all vehicles, according to 2010 data published by the trade association NGV Global. But in some parts of the world they're gaining popularity. AT&T announced last month that natural gas vehicles in its fleet now number 3,000, out of a total of more than 73,500 vehicles. By 2013, the company plans to increase the number of natural gas vehicles to 8,000.

And AT&T isn't alone. Between 2010 and 2016, the market research firm Pike Research predicts, global natural gas vehicle sales will grow by more than 9 percent year over year. Even some of the largest trucks on the road, such as long-haul UPS trucks, have been built to run on liquefied natural gas (LNG), rather than expensive diesel. And according to Rich Kolodziej, president of the trade association Natural Gas Vehicles for America, a number of companies are close to certifying conversion systems that would power big rigs using about half natural gas and half diesel fuel.

Today, most of the world's natural gas vehicles are found in Pakistan, Iran, Argentina, Brazil, and India (each country has more than one million natural gas vehicles, according to NGV Global). In Egypt, for example, virtually every taxi runs on natural gas. Conversions have been available there at no upfront cost, said Kolodziej. Instead, government and banks have subsidized the retrofits, which drivers then pay for over time through a small fee at the pump. Using computer chips and a central database, he said, the fuel nozzle recognizes cars that have received the "free" conversion.

Yet it's in the United States where Pike Research expects to see the most growth in coming years, due to the possibility of new incentives and emissions regulations. Even barring new incentives, Pike Research senior analyst Dave Hurst said he anticipates strong growth. "The cost of natural gas is expected to remain significantly lower than diesel and gasoline," he wrote in an email, "and emissions restrictions will add to the cost of diesel trucks in the medium- and heavy-duty market, which will help in the payback of natural gas trucks."

While some state and federal incentives are already available, only 0.15 percent of natural gas consumed in the U.S. goes toward fueling vehicles, according to an MIT study published earlier this year. Most of it is used for industrial boilers and residential space heating.

According to Jigar Shah, chief executive of the Carbon War Room, which researches and implements entrepreneurial solutions to climate change, the U.S. is behind the curve in transitioning to natural gas vehicles. In an interview, he blamed regulatory hurdles for adding to retrofit costs. "Here, you can be thrown in jail for tampering with your engine," he said.

Well, not quite. But the U.S. Environmental Protection Agency and state agencies do regulate vehicle emissions and certify the vehicles and engines produced by automakers. Removing or disabling part of a certified vehicle or engine can be considered tampering-a violation punished with hefty fines. "EPA has to certify that your conversion system doesn't make the vehicle dirtier," said Kolodziej. California's standards (also adopted by other states) are even more strict and complex, he added, and in total these certifications can cost the retrofit provider $50,000 to $100,000.

Fleet operators also have to make sure a retrofit won't mean sacrificing the original manufacturer's warranty. For example, BAF Technologies' EPA-certified CalComp system, which converts Ford pick-up trucks and vans to run on CNG or a combination of CNG and gasoline, is also sanctioned by Ford, thus preserving the warranty.

Pay Now, Save Later

"Theoretically, it shouldn't be expensive" to convert vehicles to run on natural gas, said Kolodziej. In many countries, he said, it's common for natural gas vehicles to simply use a three-gallon steel tank-even if it means refueling every 70 miles or so because CNG requires more storage space than gasoline. "Americans don't want to be inconvenienced." They demand more range, which means larger tanks, which in turn are made from carbon-wrapped composite materials. These tanks are lighter weight, but also more expensive, he explained, so storage accounts for about half the cost of a conversion.

For its part, AT&T expects to spend $350 million on its 8,000 converted CNG vehicles. That works out to nearly $44,000 per vehicle, compared to retail prices starting around $25,000 to $30,000 for gasoline versions (though no doubt AT&T would enjoy a bulk discount). Supported by Recovery Act funding, UPS reportedly bought 48 factory-built LNG trucks this year for about $200,000 apiece, or twice the price of a comparable diesel big rig.

And there's often an additional investment for fueling infrastructure. At this point, given the limited availability of natural gas filling stations, Kolodziej said natural gas is most compelling for fleets with "return to home" or "point to point" vehicles-vans, trucks, and buses that run predictable routes and come back to a central location, where "one station fills them all," or vehicles that travel between two depots equipped with filling stations.

The United States has about 180,000 gasoline stations, he said, compared to about 1,000 CNG stations (about half of those are open to the public). To transition to another fuel, he asked, "How do you begin? Start installing stations that you know will be profitable." One aspect of this is calculating how quickly vehicles must be refueled. A fleet of school buses that has all night to refuel, for example, can afford a smaller compressor. But a public station designed to meet the needs of an unlimited number of light-duty vehicles on demand would need a much larger, more costly compressor.

It's a different story if the main goal is to grow the market for natural gas vehicles, Kolodziej said. Germany, for example, has built a public network of about 950 natural gas filling stations in recent years with an approach that Kolodziej described as "if you build it, they will come."

Costs aside, AT&T and others are betting that natural gas vehicles will earn their keep by saving money on fuel in the long run. Last year trucks and vans fueled by CNG saved AT&T the equivalent of one million gallons of petroleum, the company says.

"The more you drive, the faster you save money," Kolodziej said. Take for example a cargo van that travels, say, 25,000 miles per year and gets 17 miles per gallon. That adds up to nearly 1,500 gallons annually. With CNG, fuel costs can drop by about $1.50 per gallon-equivalent compared to gasoline (for 2012 fuel cost calculations, the EPA assumes $3.63 per gallon regular gasoline, $3.82 for diesel and $2.07 per gallon CNG). Multiplied by 1,500 gallons per year, that amounts to about $2,200 in annual fuel savings.

The savings add up faster in cases where the vehicle travels more miles and has lower fuel economy (or, Kolodziej noted, if the fleet operator fills up at a private station where CNG is cheaper). Depending on these factors, a natural gas vehicle can pay back the cost in three to seven years. Fleet operators and drivers can see even quicker returns in places where there's a larger gap between natural gas and gasoline or diesel prices, often subsidized by government.

Natural gas vehicles also help companies reduce their tailpipe emissions, but not by enough to meet long-term climate goals, said Shah. "Natural gas is a great choice for finally doing something about oil, but most likely not the ultimate winner in 2050," given the imperative to make more drastic emission cuts, he explained in an email. The European Commission's Transport 2050 strategy, for example, calls for cutting carbon emissions from the transport sector by 60 percent by mid-century.

According to California state government figures cited by NGVAmerica, as a fuel for heavy-duty vehicles, compressed natural gas has carbon intensity only 20 percent lower than gasoline.

Power Plants vs. Vehicles

Natural gas has generated buzz as an alternative to coal for generating electricity amid the development of controversial hydraulic fracturing ("fracking") techniques for extracting natural gas. But according to Shah, "If natural gas is promoted for anything, it should be for transportation." He explained, "Natural gas combined-cycle power plants are quite cheap to build. The big cost is the fuel." If the price of natural gas in the field were to double, for example, customers would likely see comparable spikes on their electric bills, he said.

In transportation fuels, on the other hand, the price of natural gas in the field makes up a fraction of the price at the pump (according to Kolodziej, it's about one-third). The remainder comes chiefly from transporting the natural gas, as well as compressing it, taxes, and a markup for profit.

Being less vulnerable to the price of fuel in the field is especially important when it comes to natural gas, said Shah, because its price is "notoriously unreliable."

However, "We don't need to be addicting ourselves to another fossil fuel," said Shah. He sees natural gas as a temporary and partial solution for transportation. "It's not a permanent call," he said. A power plant built to burn natural gas can be expected to generate electricity for 40 to 50 years, he said. With vehicles, "you're only making that decision for 20 years. Trucks don't last forever."