New Delhi, April 19 -- After U.S. President Donald Trump and Iran's foreign minister said the Strait of Hormuz was fully open to commercial vessels after almost seven weeks of war, oil prices plunged 10% and the stock market rallied Friday.

Motorists, hoping for relief at the pump, wondered how quickly gasoline prices might fall once oil tankers stuck in the Persian Gulf were moving again. A gallon of regular gasoline cost $4.08 on average in the U.S. Friday, which was 37% more than before U.S. and Israel attacked Iran but down a few cents from a week ago.

But when gas prices spike, they don't typically drop as quickly as the cost of crude. Even if Iran keeps the waterway open in the face of a U.S. blockade of its vessels, it still coul...