BOSTON (TheStreet) -- With $4-a-gallon gas and diesel at the pumps now and their prices sure to rise, natural gas is looking like a sure bet to replace diesel as the fuel of choice for the nation's high-mileage truck fleet.Given the boom in North America's production of natural gas, there is a glut of it and it is selling at low prices not seen in a decade. Once converted to compressed natural gas (CNG) or liquefied natural gas (LNG) for use in compatible engines, it costs about 42% less than diesel on a per-gallon basis. The Department of Energy's Energy Information Administration forecasts that this difference is likely to be long-lasting. But so far, natural-gas-powered long-haul trucks haven't been a practical alternative to diesel, since there are few natural gas refueling stations nationwide. Its use has been limited to specific types of fleets, such as taxis or light delivery trucks, operating within a specific range of a fuel station. 10 Stocks That Won't Leave You High and Dry >> But that's changing as the trucking industry is the early phases of major change. For one thing, engine makers, such as Cummins CMI , already the leading natural-gas heavy-engine maker, and truck makers, such as PACCAR PCAR , are seeing such big demand from truck-fleet managers that they are shifting production capacity to build more engines and trucks that burn CNG or LNG. "We expect this sector to be a growth driver for both companies, though (it's) starting off a pretty small base at present," said Jim Corridore, an S&P Capital IQ equity analyst, in a research note. "These trucks and their engines are priced at a significant premium to diesel engines, but they can provide savings since there is such a big divergence between the cost of natural gas and the cost of diesel fuel." Others are seen to benefit as well. Clean Energy Fuels CLNE , originally backed by financier and natural gas advocate T. Boone Pickens, said in February that it has partnered with truck maker Navistar International NAV to offer customers a comprehensive engine/truck/fuel package. Customers can lease natural gas fueled trucks from Navistar (powered by a Cummins engine), as well as contract for fuel bought from Clean Energy's 257 fueling stations. Clean Energy also plans to build a network of 150 LNG truck-fueling stations along major truck routes that will allow LNG-powered trucks to travel coast-to-coast. And Canada's Westport Innovations TSX:WPT , a developer of natural-gas fuel-injection technology, is in a joint-venture deal with Cummins to design an engine, based on a Cummins diesel engine, that will burn CNG, LNG or bio-methane. It is expected to come to market early in 2013. 4 Stocks That Are Real Sleepers in 2012 >> Here are six stocks that should benefit from the trucking industry's shift from diesel fuel to natural-gas in the coming years: 1. Cummins CMI Company profile: Cummins, with a market value of $23 billion, makes diesel and natural gas engines, electric power generation systems, and engine-related products. The firm sells to original equipment manufacturers and distributors, including PACCAR and Daimler. In announcing March 21 that it has begun development of a 15-liter heavy-duty, spark-ignited natural gas engine to meet demand for on-highway applications, the company said it "believes that there is a strong market for engines powered by an alternative to diesel fuel." It has already started producing on-highway natural gas powered engines for Cummins Westport, its joint venture with Westport Innovations in Vancouver, B.C. Dividend Yield: 1.38% Investor takeaway: Its shares are up 32% this year and have a three-year, average annual return of 53%. Analysts give its shares eight "buy" ratings, four "buy/holds," seven "holds," and one "weak hold," according to a survey of analysts by S&P. Analysts estimate it will earn $10.36 per share this year and that will grow by 13% to $11.68 per share next year. S&P has it rated "strong buy" with a $152 price target, a 31% premium to the current price. 8 Fertilizer Stocks Primed for Growth >> 2. PACCAR PCAR Company profile: PACCAR, with a market value of $16 billion, designs and manufactures light-, medium- and heavy-duty trucks and aftermarket parts under the brands Kenworth, Peterbilt and DAF. Dividend Yield: 1.67% Investor takeaway: Its shares are up 15.6% this year and have a three-year, average annual return of 9.3%. Analysts give its shares nine "buy" ratings, and 12 "holds," according to a survey of analysts by S&P. S&P has it rated "strong buy," with a $66 price target, a 55% premium to its current price and says "we think the company is in the midst of a multi-year up-cycle in North America, and near the start of a new cycle in Europe." 3. Navistar International NAV Company profile: Navistar, with a market value of $2 billion, is a leading manufacturer of commercial trucks, buses, diesel engines, chassis, and military vehicles. Investor takeaway: Its shares are down 10% this year and have a three-year, average annual loss of 0.4%. Analysts give its shares 10 "buy" ratings, two "buy/holds," five "holds," and one "weak hold," according to a survey of analysts by S&P. 4. Clean Energy Fuels CLNE Company profile: Clean Energy, with a market value of $1.6 billion, sells natural gas vehicle fuels to about 320 fleet customers in North America, by owning, operating, or supplying compressed natural gas or liquified natural gas to 176 fueling stations. The company primarily builds these fueling stations for municipal fleets, such as buses and taxis, but it plans to open 70 public CNG stations by the end of the year at truck stops in 33 states and another 80 in 2013. Investor takeaway: Its shares are up 46% this year and have a three-year, average annual return of 34%. Analysts give its shares one "buy" rating, six "holds," and one "weak hold," according to a survey of analysts by S&P. Analysts estimate that it will lose 63 cents per share this year and 32 cents per share in 2013. 5. Quantum Fuel Systems Technologies Worldwide QTWW Company profile: Quantum, with a market value of $31 million, develops natural gas fuel cells and renewable energy-generation systems and services for use in industrial vehicles and power generators. It also makes high-pressure, CNG storage tanks for use on heavy- and light-duty truck fleet vehicles. Investor takeaway: Its shares are down 11% this year and have a three-year, average annual loss of 65%. Analysts give its shares one "hold" rating, according to a survey of analysts by S&P. Analysts estimate that it will lose 42 cents per share this year but turn it around next year and earn 7 cents per share. 6. Westport Innovations TSX:WPT Company profile: Westport, with a market value of $1.6 billion, develops engines that operate on compressed natural gas, liquefied natural gas, hydrogen, and renewable natural gas fuels, such as landfill gas, using its proprietary fuel injection technology. It is also in an agreement with Cummins to develop CMG-powered truck engines which will result in the manufacture of the Westport HD 15-litre engine in a Cummins factory in Jamestown, N.Y. Investor takeaway: Its share are traded on the Toronto Stock Exchange. They are down 12% this year, but have a three-year, average annual return of 80%. Analysts give its shares five "buy" ratings, five "buy/holds," two "holds," and three "weak holds," according to a survey of analysts by S&P. Analysts estimate it will lose 55 cents per share this year and turn it around to earn 88 cents per share in its fiscal year 2014. >>To see these stocks in action, visit the 6 Stocks to Benefit From Truckers' Switch to Natural Gas portfolio on Stockpickr.