If you're traveling alone, the best mode of long-distance travel is the train. Your second-best choice in terms of fuel efficiency would be the flight. Both options are energy-efficient because the fuel used is distributed across large numbers of passengers. The more passengers that ride, the more fuel-efficient the trip becomes.

For energy consumed per passenger mile, driving alone is the least optimal choice, though certainly there’s no battle for radio control in that scenario.

To figure out the most fuel efficient mode of transportation, it’s important to know how much energy is used per passenger-mile for each mode of transportation.

This quick table demonstrates the differences in various long-distance transportation modes based on government data. The energy consumed is converted into British thermal units (BTU) per passenger mile, which allows us to compare across different types of transportation (air, rail, highway), number of passengers, and fuel type (jet fuel, electricity, or gasoline).

Now you may have guessed that the correct answer to our question changes if you’ll be traveling with a buddy. The energy consumed per passenger mile for the rail and airline systems will stay the same. But the energy consumed per passenger mile for the car and SUV will be divided roughly by the number of people traveling in the vehicle.

For example, if you are traveling with just one other person, your BTU per passenger mile for the 25 mpg car will be nearly halved to 2,530. In looking at the table, the best choice is still to take Amtrak; however, the second-best option is to drive together in an economy car. The third-best option would be to fly. And the least fuel-efficient option would be to drive together in an SUV (3,914 BTU per passenger mile).

If there are three people traveling, the most fuel-efficient option is to drive an economy car (1,720 BTU per passenger mile). The second-best option now is to take the train. The third-best choice is to drive the SUV (2,635). The worst option in this case, is for the three of you to fly to your destination.

In the real world you will clearly have other variables to consider when making travel plans, including coast, time and convenience. But if energy efficiency is ever a deciding factor, keep these figures in mind.

Note: Carpooling figures are adjusted for the fact that additional passenger weight results in decreased mpg that is proportional to the weight of the car. Assume that the average 150 lb. adult passenger reduces fuel economy by, 2 percent in standard cars and 1 percent in SUVs.