As the cost of petroleum-based fuels began to increase dramatically in the 1970s, fuel efficiency started to become an important factor for car purchasers. The 1973 oil crisis became the pivot point when the members of the Organization of Arab Petroleum Exporting Countries created an oil embargo that resulted in gasoline shortages and higher prices at the pump. Car manufacturers responded to public pressure for fuel economy.
For many makes and models during that decade, new-car designs had two major traits: a less powerful engine and a smaller, lighter body. The government decided that mpg (miles per gallon) claims needed to be monitored, and the U.S. Environmental Protection Agency (EPA) began confirming the mpg estimates of automakers.
Three mpg categories are rated by the EPA under three driving conditions: city, highway and combined. Each has variables that result in different mpg numbers. City driving is characterized by starting a cold-engine vehicle and traveling in stop-and-go situations, including red lights, stop signs, and traffic jams. Highway driving is described as traveling for long distances on roads that accommodate higher speeds without frequent stops. Combined driving is a blend of both city and highway driving.
City mpg generally is the lowest mpg rating for a vehicle primarily because of the frequent starting, stopping and idling. Highway mpg typically is the highest because uninterrupted driving tends to burn less fuel. However, it should be noted that personal habits can have a positive or negative effect on mpg, even on highways. EPA mpg ratings do not include the use of accessories like air conditioners, which can significantly decrease efficiency if they are used on high output settings and never turned off. Carrying heavy cargo on roadways with steep hills and traveling at excessive speeds can result in a large drop in highway mpg. Fluctuating vehicle speeds can be troublesome as well: daydreaming drivers can speed up and slow down without cause.
Sharp differences in the three mpg ratings for a particular vehicle aren't uncommon. However, technological advances, such as the hybrid vehicles, have helped close large gaps between city and highway mpg. When it comes to expectations, we can suffer from wishful thinking and focus on the largest mpg number when purchasing a vehicle. However, you should consider under what circumstances your new or used car will be driven. If most of your driving occurs on streets with numerous traffic lights, stop signs, and slow-moving vehicles, the city mpg estimate will probably more accurate. If you drive on long stretches of high-speed interstate highways, highway mpg may be more relevant. If your driving is an equal blend of city and highway trips, the combined mpg number could be an accurate reflection of the vehicle's expected fuel consumption.