Traveling in a full plane is generally a more fuel-efficient way to go than being alone in a car. But if you have a couple of passengers in your car, cruising the highways outpaces air travel by a lot.

Traveling by water, though generally slower, is even more efficient. That's why so much big and heavy stuff moves around Hawai'i in barges. Even the proposed high-speed Superferry, which uses far more fuel for the load than a tug and barge, is more efficient than cars and planes.

Federal statistics make clear that a crucial factor in figuring out fuel efficiency is whether you take advantage of the capacity of the vehicle you're in.

The U.S. Energy Information Service notes that in 2004 the average new car got nearly 30 mph. If you assume a small car has four seats, that translates to nearly 120 "seat" miles per gallon — or a little less because of the additional load.

The agency reported that new aircraft last year were getting just short of 54 seat miles per gallon. (See www.eia.doe.gov/oiaf/ky oto3/tbl8.html.)

(Alaska Air reports getting 62 seat miles a gallon and Northwest reports 53. Aloha and Hawaiian elected not to provide figures for this report, but it is believed that short flights reduce the efficiency their aircraft might otherwise get and their numbers may be lower.)

Assume there is a bridge across the Ka'ie'iewaho (Kaua'i) Channel, originating near the Honolulu Airport and ending near the Lihu'e Airport, a 100-mile distance. You have your choice of a full car or a full plane or full ferry. And you have to pitch in your share of the fuel for the ride.

In the car, at 120 seat miles per gallon, your share would be a little less than a gallon. In the plane, at 54 seat miles to the gallon, it would cost almost two gallons.

The proposed Hawai'i Superferry will burn about 2,500 gallons on the same route and carry 800 passengers, but it's not a fair comparison to divide one into the other because the vast majority of the Superferry load is cars and cargo. For purposes of the exercise, we assume the passengers account for roughly 16 percent of the load (250 cars at 2,800 pounds each and 800 passengers at 170 pounds each). The passengers are thus responsible for 400 gallons of the fuel, or about 2 quarts per person.

Another way of looking at it is that you pay more for speed.

Another message: Try to fill every seat before you engage the starter.

If you have a question or concern about the Hawaiian environment, drop a note to Jan TenBruggencate at P.O. Box 524, Lihu'e, HI 96766 or jant@honoluluadvertiser.com. Or call him at (808) 245-3074.