A 747 burns a gallon of fuel every second. That said, it is still more efficient than an average car, to the tune of 100 miles per gallon per passenger.

Fuel efficiency or lack thereof is not really something that you judge by rate of burn per second. A Boeing 747 is capable of burning a gallon of fuel per second as well as quite a few other airliners. The 747 also doesn't always burn a gallon of fuel per second. There is a long list of factors that determine the efficiency of a particular flight. The weight of the plane is certainly very important. If you take off with half of a load of passengers and less than half capacity of fuel, the amount of thrust needed to maintain target values is going to decrease. Wind and wind resistance will also play a role here. Low level manuevering with flaps deployed burns more fuel than high level cruise flight. Favorable and non-favorable wind conditions during cruise flight will also affect the amount of thrust used. These are only a few issues to consider when determining how much fuel is burned. It varies in the same way as your car does. You'll burn more gas driving around in the city with constant stops and starts. Likewise, you will burn less when your on the highway with your cruise control set. The same is true here. The efficiency of passenger aircraft is usually measured in seat-miles. This term refers to the cost of moving one person one mile. Airlines usually keep their aircraft as long as they maintain a specific seat-mile cost and maintaince cost for the particular needs of the airline. Once it becomes too much or the airliner is costing too much in fuel economy, it will either be reassigned to a more profitable route or removed from service. Sometimes an airliner may be uneconomical for one airline but very efficient for the needs of another. Many airlines have second and third hand planes because of this reason.