Why do Americans call oil gas?

Answer 1

Over here in America most cars run on gasoline and some run on diesel fuel, natural gas, propane gas or electric.

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When you lubricate something, you say you oil it, you don't say you gas it. We call it what it is, refined into various fuels or the basic oil.

Answer 2

In the United States, gasoline is commonly referred to as "gas" as an abbreviation for gasoline. This term has been used for decades and has become ingrained in American English.