Why is gasoline suddenly under $4 a gallon?

Gas prices often ease in the fall.

One reason: It's no longer summer, which is the peak season for gas. (Fewer people drive in colder weather, AAA points out.) With less demand, there's a touch more gas on the market.

Refineries also finish their annual maintenance by late fall. So that adds a bit more gas to go around.

And the blend of gasoline that's on the market gets cheaper.

Refineries make two types of gasoline every year: summer and winter blends. The summer blend produces less smog, because it evaporates less quickly in the heat; the winter blend helps engines start up in freezing weather, and uses a bit more butane, which helps lower its cost.

(Normally, the switch happens around September 15. This year, the Environmental Protection Agency and the Department of Energy said the switch would happen on September 1, citing gasoline prices that have been driven up by the war in Iran.)

But none of those are the biggest reason why gas prices are falling, says Jorge León, head of geopolitics at the research group Rystad Energy. The main driver is the falling price of Brent crude oil, which is now down to about $90 a barrel, and which is what refineries need to make gasoline, diesel and jet fuel.

That price is falling because there's a bit more oil coming out of the Middle East via pipelines, and because market traders have rejoiced at the intensifying diplomatic efforts between the U.S. and Iran.

That progress could easily be reversed if the war escalates, León wrote in an email to NPR. "President Trump may see further military action as a way to regain negotiating leverage before Democrats take control of Congress in January. If we see renewed escalation, much of last week's oil price decline could quickly reverse," he wrote. The same would be true for gas prices.

Congressional Democrats have expressed their support for ending the war in Iran. While they have limited power to do that, they do control the power of the purse, and can withhold additional war funding or hold back confirmations of key officials.

For now, if the price of Brent crude keeps falling, so could prices at the pump. "However, this does not mean a complete 'normalization,' a return to the price levels we had before this conflict started," Jaime Brito, executive director of refining and oil markets for Dow Jones Energy, wrote in an email to NPR.

To get back to pre-Iran war gas prices, the whole oil supply chain would have to recover. Ships that have been running long alternative routes to avoid the Strait of Hormuz would have to return to their points of origin, then resume their normal routes, taking crude oil to refineries. Those refineries would have to pump out enough fuel to alleviate global demand — and depleted stockpiles. The whole process could take months after a lasting peace deal.

"Coming back to those levels will require a return to normal crude production, shipping and refining of the entire global supply chain," Brito continued.

And while $4 a gallon may seem like a bargain these days, in an update on its website, AAA said that gasoline prices are still the highest they've ever been at this time of year.