President Trump says the war in Iran will be over soon, bringing quick relief to Americans facing average gas prices that are now more than $0.64 higher than this time last month. But history suggests that any eventual return to normalcy at gas pumps is likely to be a drawn-out process.

Previous presidents, most recently Joe Biden, know the pattern well.

As the Federal Reserve Bank of St. Louis explained in 2022, crude oil and refined gas prices don't always move in tandem, a phenomenon known as "rockets and feathers."

They often shoot upward together (like a rocket), but when crude oil prices later fall, gasoline prices sometimes drift down at a much slower rate (like a feather).

The concept has already been in limited evidence in recent days.

The rocket side of the equation was clear last week when hostilities began and the price of crude oil — both international benchmark Brent crude (BZ=F) and US benchmark West Texas Intermediate (CL=F) — shot up. Pump prices went up quickly with them.

But on Monday and Tuesday, crude oil prices fell 25% from a late Sunday peak — a change that hasn't yet been reflected at the pumps.

In fact, average pump prices have moved up in that time period, according to the American Automobile Association (AAA). The average cost per gallon was $3.47 on Monday, $3.53 on Tuesday, and is now $3.57 on Wednesday, according to the group.

That hasn't changed the White House narrative. Press secretary Karoline Leavitt told reporters on Tuesday that when hostilities end, "Americans will see oil and gas prices drop rapidly, potentially even lower than they were prior to the start of the operation."

Read more: What an extended war with Iran could mean for gas prices

A longtime political pain point

The formal economic term for the phenomenon is "asymmetric pass-through" and stems from a variety of causes, including the lag between refiners buying crude oil and then selling their refined product and the need to preemptively protect bottom lines in moments of uncertainty.

What it means in practice is that drivers are often the last to feel relief when oil shocks ease. That also means outsized political costs with voters angry about the lingering price increases, which flared during the Biden administration.

It was in the aftermath of Vladimir Putin's invasion of Ukraine in 2022 that both crude oil and pump prices spiked. Prices at the pump then stayed elevated even after crude oil prices eased, frustrating Biden and his aides for months.

In a March 16 briefing that year, then-Press Secretary Jen Psaki said, "many accept that gas prices rise quickly but fall slowly — the so-called 'rockets-and-feathers' phenomenon — but President Biden rejects that."