New cars have the fuel efficiency listed in large, bold letters -- one for city fuel efficiency, and one for highway efficiency -- on the window sticker. The highway number is higher than the city number, which means that the car can drive further on a gallon of gas on the highway. This is not to say that highway driving uses less gas during a certain amount of time -- a minute of highway driving will burn more fuel than a minute of city driving -- but that for the same amount of gas, driving on a highway will get you farther down the road.
The ideal speed to maximize mpg for most cars falls within a range between 50 and 65 mph. When engines rev up and reach higher gears, the efficiency of their output increases until they are working so hard that a marginal increase in output requires a large amount of extra fuel. Since wind resistance and friction also increase at higher speeds, fuel efficiency drops off fast once the engine reaches its peak efficiency level. The peak level will vary for each car depending on the engine, the weight of the car, its aerodynamics and many other factors. In general, the lighter a car and the larger the engine, the higher the speed it will take to reach peak efficiency. Since light cars usually have smaller engines, they often peak out around the same speed as large cars with larger engines. Sports cars with large engines will tend to see peak efficiency at higher speeds.
Another important reason that highway driving is more efficient than city driving is that it requires less waste of momentum in the form of braking. Every time a driver stops, he must burn extra gas to accelerate back to the speed he was driving before he stopped. Put simply, it takes less gas to maintain speed than it does to achieve that speed. Another advantage to highway driving is that gas can be conserved when going downhill by coasting, whereas in the city, coasting will usually be interrupted by turns, stop signs or stoplights. Drafting behind other vehicles can also increase highway fuel efficiency, but it is a dangerous practice and should not be attempted. In order for drafting to be effective, you'd have to follow close behind a large vehicle -- closer than the mandated safe following distance.
