The prices of so-called distillate fuels like heating oil and diesel have been plunging lately, and diesel now costs less than gasoline in 21 states, according to a recent article in the Wall Street Journal. Meanwhile, wholesale heating oil prices have fallen by 40 cents per gallon over the last two months to $1.45 a gallon, according to the U.S. Energy Information Administration.

The increase in distillate inventories, which are up 12 percent from a year ago, could be a boon to households that use heating oil as their primary heating fuel, according to the Journal.

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, the article states.

"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," the article states.

"This is the cheapest I've paid for diesel in a long time," April White, a systems engineer in Alameda, Calif., told the Journal. "Because of how much cheaper diesel is, I just kind of fill up without thinking about it." Heating oil is virtually identical to diesel but costs less because it is untaxed.

"Falling diesel prices suggest that the world's crude-oil glut is now becoming a glut of refined products. Analysts and investors say increasing petroleum-product inventories could keep prices subdued into 2016," the Journal wrote.

In recent years, refiners have invested heavily in new capacity to make a range of products known as middle distillates, including diesel, jet fuel, heating oil and kerosene, the article states. "The widespread expectation was that developing countries would need more of these industrial fuels, while industrialized nations would become more energy-efficient and use less gasoline." Refiners are now competing to sell their diesel onto the global market, pushing down prices.

Though diesel is mostly associated with trucking fleets and farmers, a small percentage of Americans own diesel cars, which are touted as cleaner and more fuel-efficient.

To read the Wall Street Journal article, click here.