PORTLAND, Oregon (TheStreet) -- Driving around my area this Memorial Day Weekend was an expensive proposition. Here in the great state of Oregon it was hard to find unleaded gasoline for less than $4.19-a-gallon.

That means to fill a 15-gallon fuel tank cost around $63, and I'm aware there are many states where the cost is significantly higher.

We're spending a fortune burning a petroleum based fuel and the cost is projected to go even higher in the months and years ahead.

The good news is that many nations of the world including America are making progress by making a shift towards vehicles that burn natural gas instead.

According to the organization

Natural Gas Vehicles for America,

there are about 120,000 natural gas vehicles (NGV) on U.S. roads today and more than 14.8 million worldwide.

There are about 1,000 NGV fueling stations in the U.S., and half of them are open to the public. That number is expected to grow significantly in the next 24 months.

Last year, a former officer of the publicly-traded company

EOG Resources

(EOG) - Get Report

drove across America with a NGV-powered Hummer to prove it is possible.

The fuel cost of the trip was almost half of what it would cost using regular unleaded gasoline.

Honda Motor

(HMC) - Get Report

offers a Civic that runs on compressed natural gas (CNG). In fact, it's the only factory-built CNG car in the country available to non-fleet customers. It's available in 36 states so far and the review by

Car and Driver

for the 2012 model is a fascinating read.

The CNG Civic has many advantages, including a database of publicly accessible CNG refueling stations and, of course, it's cleaner-burning and gets better mileage than a comparable gasoline guzzling vehicle.

Yet, as Car and Driver states, another big advantage is the almost ludicrous difference between the price of a gallon of petroleum-based fuel and a gallon of Compressed Natural Gas:

"The national average for CNG in January 2012 (the most recent month of the EPA’s regular reports) was $2.13 per equivalent gasoline gallon.

"In EPA highway figures and using the most recent national average price of $3.52 per gallon for regular gasoline, the Civic LX and hybrid cost 9.0 and 8.0 cents per mile compared with just 5.6 cents per mile for the CNG."

Now with gasoline prices heading back toward $5-a-gallon, and even with CNG prices rising to $2.50 per equivalent gasoline gallon, the savings per mile is getting close to 50%.

There's another added bonus -- the good feeling drivers of CNG vehicles receive from knowing that the carbon emissions from burning natural gas are far less polluting to the air we breathe.

For investors, the opportunities are represented by the companies that are making the refueling stations, the engines that run on compressed or liquid natural gas (LNG) and energy companies that provide LNG.

As you may know, there's one company,

Cheniere Energy

(LNG) - Get Report

that engages in the ownership and operation of liquefied natural gas receiving terminals and natural gas pipelines in the Gulf Coast of the United States.

Their story is compelling, but the price-per-share for a company with long-term plans to finally turn a profit makes it more worth learning about then investing in.

The investment theme that is more exciting to me involves the companies that make the refueling stations and the engines themselves.

In particular, I'll be keeping an eye on

Clean Energy Fuels

(CLNE) - Get Report

and

Westport Innovations

(WPRT) - Get Report

.

Clean Energy Fuels was mainly the brain-child of legendary billionaire energy investor T. Boone Pickens, who along with some other investors wanted to start a company that provides natural gas as an alternative fuel for vehicle fleets in the United States and Canada.

The company designs, builds, operates, and maintains fueling stations, as well as supplies compressed natural gas (CNG) and liquefied natural gas (LNG) fuel for medium and heavy-duty vehicles. Its CNG is used in automobiles, light to medium-duty vehicles, refuse trucks, and transit buses as an alternative to gasoline and diesel.

Clean Energy Fuels also sells non-lubricated natural gas compressors and related equipment used in CNG and LNG stations; and produces renewable natural gas, which is used as vehicle fuel or sold for power generation.

If you study their well organized, highly informative

it's hard not to become a convinced believer in the company's mission, goals and promising future. That being said, it hasn't made any net profits yet, so my suggestion is to wait until the stock market has its next deep panic attack and then pick up some shares near the 52-week low of $9.02.

Westport Innovations

is another exciting and promising enterprise. Like Clean Energy Fuels, Westport has a legion of zealous believers and supporters. Their well-founded enthusiasm is contagious.

Once again, here's a focused company with outstanding potential, but it will take another year or more before they start seeing profits. You can read all their exciting developments and their first quarter 2012 earnings report by going to the

Westport news page

.

Speculative investors and traders may again want to wait for the company's share price to revisit or test their 52-week low of $19.66.

You can see from the one-year comparative chart below that WPRT and CLNE tend to move in lock-step.

My guess is that sometime in the next four years, we're going to see a major push forward on both CNG and LNG as the fuel of the future. We may even see government subsidies and grants.

In the meantime make this investment theme one you'll keep a careful eye on. Until we get closer to CLNE and WPRT becoming profitable I suggest just being an informed and patient observer.

Disclosure: At the time of publication, Marc Courtenay held no positions in any of the securities mentioned in this article.