Cars are not supposed to be a good investment. The rule of thumb says that a car loses at least 10 percent of its value the moment it leaves the dealership. But soaring used-car values are turning that formula on its head for many fuel-efficient vehicles, which are suddenly in high demand as Americans fret about escalating gas prices and do not want to pay for a new car. Used cars overall are retaining a higher percentage of their original value than ever before.