According to the IEA's fourth quarter issue of IEA Energy, 61% of the world's natural gas vehicles (NGVs) are located in only four countries. India and China make up 16% of the NGVs in the world -- yet NGVs make up only 3.5% of India's total cars, and a paltry 1.2% of China's.

Where does the U.S. fit in? The world's #1 car market has fewer NGVs than Peru.

Those four countries with 61% of the NGVs? Pakistan, Iran, Argentina, and Brazil. The OECD countries have only 7% of the total, with tiny Italy commanding two-thirds of that 7%. The point? NGVs have almost nowhere to go but up from here, and the explosive potential growth -- per the IEA, NGVs will nearly double total fuel consumption from 2012 to 2018  -- will have a larger impact on oil demand than all EVs over the next five years. 

Tesla's (TSLA -2.71%) "top-down" approach to developing electric vehicles (EVs) has created massive appeal in the premium car market, but the American consumer is yet to show any real sustainable interest in NGVs -- so far -- as the benefits of cheaper fuel come with losses in range, performance, and access to fuel. But the investing story doesn't seem to lie with the actual builders of the NGVs, which in most cases are those traditional automakers. The growth opportunity looks to be in the companies that are developing the two key cogs in the machine: refueling infrastructure and engine technology.

As to the refueling network, Royal Dutch Shell (RDS.A)Clean Energy Fuels (CLNE 1.29%), and Integrys Energy Group (NYSE: TEG) subsidiary Trillium LNG are all involved in efforts to establish refueling points for NGVs. Let's take a closer look at the fuel providers and see what we can learn.

Supercharger station. Source: Jeff Cooper via Wikipedia

If you build it? Not exactly...
Unlike Tesla's efforts to reinvent the car, NGVs are largely just traditional vehicles with a different engine and fuel tank that can use either CNG (compressed natural gas) or LNG (liquefied natural gas). But exactly like the challenges that Tesla and traditional carmakers General Motors and Ford have had, the roadblock for NGVs has been all about access to energy. 

Tesla is doing something about this for EVs, with its Supercharger network of high-speed recharging stations along busy corridors. This system is completely free to use for Tesla Model S owners. Further, each station gets the power it needs via solar panels, not relying on electric utilities, though the units are on the grid to meet peak demand needs. With the electricity being essentially "free" outside of minimal maintenance costs, it's relatively affordable for Tesla to provide its Superchargers at no charge -- especially since most Tesla owners will rely on home or work for most charging needs, and Superchargers only when traveling. 

CNG refueling station. Source: Clean Energy Fuels

Automakers can't do this for NGVs, for obvious reasons, meaning demand will have to grow for infrastructure to get built. And with very little growth in real numbers expected in North America for NGVs in the near future, the companies above have taken the approach to focus on commercial transit and fleets. While the total market is smaller, there are a few big reasons why fleets are better targets than the daily commuter:

Fuel cost is a major concern for shipping fleets, and each truck can use 10,000 gallons per year in fuel or more. Since most shippers and commercial transit operators follow predictable, regular routes, providing access to fuel is greatly simplified. This small group -- which consumes a lot of its product -- is where Clean Energy Fuels and Integrys Energy subsidiary Trillium are focusing, and where Shell's efforts with partner TravelCenters of America are pointed. 

Educating the customer to drive demand
Both Shell and Clean Energy Fuels have worked with Westport Innovations to show trucking fleet owners how natural gas-powered trucks can lower fuel costs substantially, and how engines from Westport and partners like Cummins and Volvo meet their power and reliability demands. Clean Energy Fuels already operates hundreds of fleet customer fueling stations and is building out at least 150 public stations that will serve LNG for over-the-road truckers. Similarly, Shell and TravelCenters are building "up to 100" LNG stations at existing TA truck stops. Both Shell and Clean Energy have worked with Westport to spread the word, presenting at trade groups, meeting with large customers, and in the case of Clean Energy and Westport, offering financial incentives to fleets to make the jump to CNG-and LNG-powered trucks. 

Trillium is also opening stations and working with fleet owners to teach the benefits of natural gas versus diesel. However, the Integrys Energy subsidiary has chosen to only offer CNG, counting on enough fleets not needing the longer range and much faster speed of filling that LNG provides for the vast majority of their trucks. Shell and Clean Energy -- already the largest LNG and CNG refueler in North America -- will be able to provide fleet owners both LNG and CNG. 

For investors, it's about growth potential
Tesla's recent earnings announcement adds more evidence that it will continue to dominate the EV conversation, while growth in NGVs will occur in public transit and trucking for now.

Of the refueling side, there's direct upside with Clean Energy Fuels. It's run by founder Andrew Littlefair and counts its other founder, T. Boone Pickens, as its largest shareholder and a board member. Both made large stock purchases in 2013, and there are several catalysts for 2014, like new engines from Cummins Westport, and news that its existing customers ordered 70% more NGVs in 2013 than 2012. It has heavily invested in NG for transportation, and the trucking industry is beginning to adopt. 

Trillium is just a small part of a larger energy company in Integrys Energy Holdings, while Shell is one of the behemoths of the energy landscape already. The point is, the success of both of these companies is largely tied to many things besides just this one piece of the market. Both have value, but NG for transportation isn't why you should own either.