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The world is awash in diesel, sending prices plunging and delivering some commuters a rare bounty.

Diesel at the pump was cheaper than regular gasoline in 21 states Wednesday (July 22), according to auto club AAA. The national average price for retail gasoline, at $2.75 a gallon, was 4.6 cents below diesel, compared with an average discount of 37 cents from 2010 to 2014. The national average gas price last exceeded diesel’s in 2009, AAA said.

Diesel fuel typically costs more than gasoline because U.S. refineries export much of their diesel output, leaving less available for the domestic market, and federal taxes are higher for diesel than for gasoline.

As gasoline demand has risen around the world, refineries are running at high rates to produce as much of it as possible. Facilities from Russia to China to the U.S. make diesel alongside gasoline, so in their rush to produce gasoline they are churning out diesel faster than the world wants to consume it.

“This is the cheapest I’ve paid for diesel in a long time,” said April White, a systems engineer in Alameda, Calif., who fills up her diesel-powered Volkswagen Golf about once a month. Diesel prices in California cost an average of 72 cents a gallon less than regular unleaded gas prices on Wednesday, according to AAA.

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