The price of gasoline is declining more slowly than oil, illustrating how consumers aren’t fully benefiting from falling commodities prices.

Typically, gas prices go up and down in tandem with crude oil, the main ingredient. But recently, oil prices have been falling much faster than gas prices. Drivers have paid at least $1 billion more for gasoline than they would have if the historical pattern had continued this year, according to government and industry statistics.

Regular...