European finance ministers — also known as “the gang that couldn’t shoot straight” — surprised investors by finally getting their act together, jointly cobbling a relief package for Greece that should remove any sort of doomsday risk from the market, at least for the next few quarters. A powerful relief rally took place in stock markets across the globe, as a sign of a welcome reaction.

Yet the good news also brings an unexpected problem. A stabilized Europe, coupled with a U.S. economy that is likely to dodge a recession, means oil prices are starting to perk back up. Crude oil, of the West Texas Intermediate (WTI) kind, shot up from $77 a barrel to $93 in October and, before long, expect to see gasoline prices quickly move back to the $4 mark.

#-ad_banner-#Meanwhile, even as crude oil surges, natural-gas prices remain in a funk as U.S. output surges ever higher. The broadening price differential between crude oil and natural gas could be a boon for several companies, and as I mentioned a couple weeks ago, is bringing real attention to the exploration side of the business. But another group of companies is positioned to benefit if natural gas takes off as an energy source in the transportation sector.

We’ve already got evidence of how such a move would be greeted by investors. Westport Innovations (Nasdaq: WPRT) has seen its stock nearly double since February as a rising number of truck fleet operators line up to use Westport’s natural gas retrofit technology. Considering the high price of diesel fuel, a switch to natural gas makes ample sense. Reducing oil imports (from sometimes hostile trading partners) and boosting usage of U.S.-produced gas also holds appeal in terms of national security and persistent trade imbalances.

What about cars? Well, the same logic applies. On an apples-to-apples basis with gasoline, natural gas costs less than $2 per gallon. (It currently costs upwards of $5,000 more for a natural gas-powered car than a regular car, though that should drop as production volume rises.) Yet consumer awareness of the benefits remain fairly low and Congress — which had been expected to be a big backer of natural-gas transportation subsidies — no longer looks set to provide any help. Another concern: a shortage of natural-gas fueling stations. Luckily, a pair of companies is stepping into the breach to help catalyze the industry transition.

Clean Energy Fuels (Nasdaq: CLNE)
This company, formerly known as Pickens Fuels, was founded by oil legend T. Boone Pickens. Though he made his fortune in crude oil, he’s become much more bullish on natural gas in recent years, especially as a transportation fuel source. He currently controls about 4.7 million shares (or 7%) of the company, so he’s got an obvious agenda. And his vision has not yet delivered the shareholder returns he had hoped. Even though Clean Energy Fuels is the largest operator of natural gas filling stations (with almost 250 at last count), bottom-line results haven’t added up. The company has never generated positive free cash flow and has had to repeatedly sell more stock to raise capital in order to extend the filling station network.

Yet that network of filling stations may has finally reached critical mass. The company is expected to book quarterly GAAP profits by the middle of next year. Fuel sales shot up 60% in 2010 (to $212 million) and should top $350 million by next year. At a $500 million run rate, the company thinks it would generate solid levels of profitability.

Of course, more natural-gas–fueled cars on the road would instantly change the company’s trajectory. And help may be on the way. Honda (NYSE: HMC) already sells a natural-gas–powered Civic, GM (NYSE: GM) is launching a line of pick-up trucks powered by CNG (compressed natural gas), and Ford’s (NYSE: F) Transit Connect minivan will soon be available in a similar configuration. European automakers, including Fiat, already sell natural-gas–powered cars in their home region.

Another way to play the trend: Fuel Systems Solutions (Nasdaq: FSYS), which has been around for more than 50 years and generates more than $400 million in annual sales. The stock has fallen more than 40% from its 52-week high to a recent $23 as the company’s Italian operations took a big hit from the end of gas engine conversion subsidies from the Italian government. For 2011, sales are on track to fall about 5% to around $407 million. Yet analysts spy a rebound in 2012 as new markets in Asia and Latin America expand. Sales are expected to rise more than 10% to around $450 million and earnings per share may double to around $1.20. Fuel Systems is also gaining traction in the United States. Those GM pick-up trucks, noted above, will be converted by Fuel Systems’ IMPCO division.

Risks to Consider: Natural gas as a transportation source only makes sense in an era of pricey crude oil. If the price of oil falls back, perhaps to below $70 a barrel, then any momentum this nascent industry may have will be blunted.

Action to Take–> These stocks are well off of their highs, in large part due to lofty growth expectations that have failed to materialize. Yet a move toward more natural-gas–powered vehicles in 2012 would quickly bring investor attention back to these lagging stocks. As oil prices move back higher, that becomes a rising possibility.