Oil prices have fluctuated up and down for months along with news about the Iran War. The price of the fuels that are made from oil have moved mostly in one direction-up-and show little signs of returning to earth soon.

The diesel fuel that powers trucks and farm equipment has gotten particularly expensive, with wholesale prices jumping 109% this year. Diesel prices at the pump hit $5.47 on Tuesday, up 7% in the past month and 48% in the past year.

American refiners are earning record margins for pumping diesel out-triple the per-barrel profits they were making at this time last year, according to data from OPIS, which is owned by the same parent company as Barron's.

The spread between the price of West Texas Intermediate crude and diesel futures hit $101.86 on Monday, the first time the margin has hit triple digits. The biggest beneficiaries include U.S. refiners like Valero Energy, Marathon Petroleum and Phillips 66, all of whose stocks have reached all-time highs in the past few weeks. Major oil companies with refining arms like ExxonMobil are also profiting.

To see why, it's important to understand the difference between the crude oil market and the fuel market.

The war has caused the largest disruption to the crude market in history, because Iran has blocked the Strait of Hormuz, and with it about 20% of the world's oil. But the market for fuels made out of crude has been even more disrupted.

The Iran War isn't the only issue impacting fuel exports. Russia's fuel exports are down too, because Ukraine has attacked Russia's refineries with drones. The attacks knocked 2.8 million barrels of capacity off line as of July, Bank of America says. To maintain enough supplies for its own citizens, Russia has restricted diesel and gasoline exports until next year. In addition, China has reduced fuel exports to make sure there's enough available for domestic use.

That has left the U.S. as the exporter of last resort. But even with U.S. diesel exports hitting record weekly levels this month, it's not enough to supply the whole world indefinitely.

The shortage comes at a bad time. The world's reserve of fuels is already low, so there isn't as much stocked away in storage tanks that can be used today.

Diesel demand is likely to spike more in the next several weeks as harvest season accelerates, notes Bank of America analyst Michael Widmer.

"In short, absent a meaningful supply recovery, the diesel market appears poised to stay tight, volatile, and expensive well into next year," Widmer writes.

There's no easy fix to the fuel shortage in the near term, absent a full resolution to the Iran and Russia conflicts. More refineries have been closing lately than opening, writes Melius Research analyst James West. Even though electric vehicle sales have begun to eat into gasoline and diesel demand, it's not happening fast enough to balance the market.

"The global refining fleet is basically flat to shrinking through 2027 before it starts growing again into the end of the decade," he writes. "That's a very different picture from the demand side, where growth is slowing but still positive."

West expects the next wave of refinery openings to come between 2028 and 2030, which could bring relief to the market. That will leave the world undersupplied for an extended period.

"We expect structurally higher refining margins over the next two years," he writes.