Fuel efficiency in any vehicle depends on many factors. Most of them are fixed by the choice of vehicle and body style: Not surprisingly, smaller, lighter, sleeker cars use less energy to move around than larger, heavier, blockier vehicles.
A study of increases in auto efficiency found that from 1980 to about 2010, most of the improvements went toward faster acceleration and heavier cars--with increases in gas mileage coming only when mandated by increases corporate average fuel efficiency (CAFE) standards. The move from sedans and station wagons into crossovers and sport-utility vehicles, for example, hardly helped the gas mileage of new family vehicles.
These days, new vehicles use a number of technologies to boost the distance traveled on each gallon of gasoline. They include turbocharging smaller engines, which lets them run more efficiently when they're not delivering maximum power--but still offer the torque and speed of a larger engine when needed. Then there are hybrid and plug-in electric cars, all of which recapture energy that would otherwise have been wasted in braking, and use it to recharge a battery pack that powers one or more electric motors to help move the car. Hybrids run only on gasoline--just more efficiently--while both plug-in hybrids and electric cars have batteries that can be recharged by plugging them into the wall.
Other techniques for making cars fuel-efficient include making them lighter, with materials from high-strength steel through aluminum to carbon fiber, and reducing their wind resistance through painstaking attention to tiny details of the body shape that minimize aerodynamic drag. Multi-speed transmissions are another technology; whereas three- and four-speed automatics were the norm just 10 years ago, we're now seeing eight- and even nine-speed automatics in volume cars. Diesel engines, too, make cars more efficient on the highway, although they're not as good in stop-and-go city traffic as hybrids are.
Finally, an equally important factor in fuel efficiency is driving style. The old advice still applies: You'll get the best fuel economy if you drive as if there's an egg between your foot and the accelerator. Accelerate slowly, plan ahead, watch the traffic several car lengths beyond you, and take maximum advantage of coasting down to a stop where it's safe to do so.