Because they make more money that way. The price for a gallon of gas listed, $3.49 9/10, is rounded up to the next penny. But most customers don’t pay any attention to that fraction, focusing instead on the standard three-digit price, falling for a classic psychological trick that makes customers think they’re paying slightly less for something than they really are. It’s the same psychology behind why things that cost 99 cents “feel” so much cheaper than something that costs $1.

The fractional pricing practice began during the Great Depression. To save money, people were driving less, and the demand for gasoline. Gas station chains introduced premiums to attract customers, such as free glasses, candy, and ashtrays, for example. Another way a gas station could differentiate itself from the competition: lower prices. Undercutting the other guy by even a fraction of a cent was enough to lure customers. The 9/10^th became permanent in the 1970s. During that decade’s oil crisis and subsequent gas shortage, the federal government stepped in to regulate gas prices, setting them with elaborate but precise formulas, which often ended in 9/10ths. The idea simply became a part of American life after that.