In the past decade, a dark horse has emerged as the most energy-efficient mode of travel: aviation. Since the 1970s, the airline industry has cut its per-passenger, per-mile consumption 75 percent, enough that travel by air now takes less energy than by car, on average. The gains outpace those of buses, trains, and auto­mobiles. An electric plane, such as the Airbus E-Fan, is perhaps the logical next step for an industry that saves energy any way it can.

Efficient Planes Have:

Better Aerodynamics The more smoothly a plane slices through the atmosphere, the less fuel it needs to get from city to city. Winglets—the little fins at the tips of the wings—break up airflow and reduce drag on a plane. Boeing found that its winglets increase efficiency by 4 percent, a big enough gain that some airlines are retrofitting existing fleets with them.

Optimized Engines The key to an efficient engine is to maximize air compression for more thrust. Design improvements have allowed airplane engines to squeeze air through more easily, and new materials enable them to withstand the higher temperatures created by more-efficient thrusters. The result is a lighter engine that gets more force out of less jet fuel.

Less Weight Even small cuts in weight make a difference. For example, instead of riveting many sheets of aluminum together, Boeing is constructing its 787 Dreamliner body out of just a few segments, each molded from a lightweight composite material. The design requires 50,000 fewer fasteners per segment and thus is a little lighter.

More Fliers Airlines have another way boost their per-passenger efficiency: more passengers. By reducing the number of flights and squeezing more seats onto planes, airlines have increased the number of people on each plane in recent years.

Car Travel Automobiles have steadily increased in energy efficiency since the 1970s, albeit at a slower rate than airplanes. The improvement largely comes from car design; automakers have tweaked exteriors into more aerodynamic shapes and adjusted engines to use less fuel. Katie Peek and Todd Detwiler/Popular Science

Intercity Buses Intercity buses--think Greyhounds--have actually seen efficiency losses. That's due in part to wasteful practices such as idling at stops. And since companies don't replace buses very often, more-efficient designs also take a long time to reach the road. Katie Peek and Todd Detwiler/Popular Science