In some ways, the term fuel-efficient transportation eludes a precise definition.

There are distinct advantages and challenges for each of the myriad ways of moving things from one place to another. Some are better for moving freight; others are better at moving people. Some are practical; others less so. Some use little fuel but take only one passenger. Others can move lots of passengers but use a lot of fuel. The multiple factors associated with each makes it difficult to compare cars, trucks, ships and railroads.

Nevertheless, some generalizations about fuel-efficient transportation can be made. We'll get to those in a moment.

A metric commonly used to measure fuel efficiency is passenger miles per gallon. This measurement takes into account the number of passengers moving over a certain distance for each gallon of fuel consumed.

For instance, a car that gets 26 miles per gallon but typically transports only one person at a time would measure 26 passenger miles per gallon. On the other hand, a bus that gets five miles per gallon but carries an average of 14 passengers would measure 70 passenger miles per gallon.

There are similar metrics for measuring the volume of freight moved using one gallon of fuel.

To gain a better appreciation for the fuel efficiency strengths and weaknesses of different transportation methods, let’s take a closer look at a few of the major ones:

Rail

According to the Association of American Railroads’ April 2010 study entitled “Railroads: Green From The Start,” U.S. freight railroads in 2009 moved 1 ton of freight for an average of 480 miles per gallon. Since 1980, railroad fuel efficiency has risen by 104 percent.

Railroad freight volume has doubled since 1980, but the amount of fuel used to move that freight has remained constant, according to the AAR report.

When compared to trucks, railroads on average are four times more fuel-efficient, AAR says. Taking just 10 percent of long-distance freight from trucks and moving it by rail would create more than $1 billion in fuel savings each year, the AAR notes.

More recently, electric trains have been making even more strides in fuel efficiency by capturing the energy creating through braking mechanisms and redistributing it as electricity to be used in the overhead lines that run above the track.

Ships

The RMS Queen Elizabeth 2 cruise ship required one Imperial gallon of fuel to move 49 feet (that translates to 1 U.S. gallon to move 41.2 feet) in the open seas. That would equate to about 128 gallons per mile or 0.0078 miles per U.S. gallon.

Taking into account the ship’s capacity of 1,777 passengers and 1,040 crew members, the QE2 (which was retired in 2008) achieved a fuel efficiency measurement of about 22 passenger miles per gallon (0.0078 times 2,817).

Not too great, but better than the one-time rumors that it took one gallon of fuel to move the ship six inches.

Automobiles

Fuel efficiency in automobiles seems to be improving every year. New technologies help cars get more and more miles per gallon. However, most cars only carry one person at a time and that brings down the average passenger miles per gallon.

According to the U.S. Environmental Protection Agency (EPA), the most fuel-efficient vehicle available in the 2010 model year was the Toyota Prius with a reported 51 miles per gallon for the city and 48 miles per gallon for the highway.

The average car in the United States typically gets between 15 and 40 miles per gallon.

Trucks

Trucks come in several shapes and sizes and, as may be expected, the larger the truck the more fuel it requires.

According to a 2004 study from the National Commission on Energy Policy, tractor-trailer trucks can get a base fuel economy of 5.3 miles per gallon. Platform trucks, delivery vans and super-duty pickups can get about 7.8 miles per gallon. And large pickups and SUV’s can get about 13.9 miles per gallon.

Tractor trailers use the bulk of fuel among all truck classes, but the U.S. government recently unveiled new standards that will be aimed at improving their fuel efficiency and reducing emissions by 20 percent.

Buses

One study out of Santa Barbara, Calif., found that a diesel bus could get about six miles per gallon of diesel fuel. If all 55 seats in the bus were filled, that would translate to 330 passenger miles per gallon.

That sounds great, but the average U.S. bus occupancy as of 2001 was just 10.75. That would translate to an average of 64.5 passenger miles per gallon.

Even at that lower number, buses outrank automobiles in terms of fuel efficiency.

Airplanes

When looking at the passenger miles per gallon, airplanes appear to be somewhere in the middle of fuel-efficient modes of transportation.

According to the U.S. Bureau of Transportation Statistics, domestic airline travel averaged 58 passenger miles per gallon in 2008. That’s about 8 passenger miles per gallon better than the 1998 levels.

For international airline operations, the numbers are a bit worse: planes recorded 47 passenger miles per gallon.

When looking at faster jets, the numbers become less efficient. A Concorde jet with a Rolls Royce/SNECMA engine typically gets just 17 passenger miles per gallon.

So, the numbers vary according to the aircraft, but what may be more concerning is that airplanes have not made significant fuel-efficiency improvements in more than a decade.

Closing thoughts

So, overall, rail service appears to be the most fuel-efficient transportation, especially for moving freight over long distances. Automobiles continue to improve and tractor trailers may be seeing progress in the near future as well, thanks to Uncle Sam. Buses can boost their numbers by increasing ridership. Airplanes, however, have not gotten much better over the years but that just means they have lots of potential to improve.

Got more thoughts on fuel-efficient transportation? Did we forget anything? Please leave us a note in the comments below.