CARACAS, Venezuela—In this dilapidated tropical capital, most prices go up monthly, and essentials like milk and flour are hard to find. But one item is ubiquitous, and practically free: gasoline.

Premium gasoline in Venezuela costs 5.8 U.S. cents a gallon, using the official exchange rate. At an informal currency exchange rate that prevails for most transactions, it is even cheaper: 1.5 cents a gallon.

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