(Susana Gonzalez/Bloomberg)

Oil prices have fallen this month to their lowest point in years, but fuel costs haven’t fallen nearly as quickly.

While West Texas Intermediate crude, a U.S. benchmark, has dropped 15 percent in the last month, the cheapest it has been since 2009, prices at the pump have slipped just 3 percent. A barrel of crude cost $42.61 Tuesday afternoon.

The national average for a gallon of regular-grade gasoline is now $2.66, according to AAA, still much cheaper than last year but well above what a gallon cost last winter.

The gulf owes to a handful of factors — more people drive in the summer, for example, and the blend of gasoline produced this time of year is costlier to make — but gas prices have also been pushed up by trouble at a refinery in Indiana, industry followers say.

Machinery issues at BP’s refinery in Whiting, Ind. — the nation’s largest outside of the Gulf Coast — have sent gas prices in the Midwest, including Illinois and Indiana, soaring by more than 50 cents in a week, said Michael Green, a spokesman for AAA. That was enough to push up the national average this week, ending a 27-day streak of falling prices.

But “just because the national average is jumping doesn’t mean that everybody’s prices are going up,” Green said.

Crude oil prices have been pushed down by high production, concerns that emerging economies aren’t growing as fast and expectations that Iran will export more oil once sanctions are eased. The U.S. Energy Information Administration expects crude oil prices to begin to increase next year but stay at lower-than-usual prices.

The refinery’s impact highlights the divide between the cost of crude oil and prices at the pump, said Tom Kloza, head analyst at the Oil Price Information Service. Gasoline can’t be stored as easily as crude oil, so hiccups in the supply chain can quickly hike up prices, making for a market prone to big swings.

Gasoline blends in the summertime are also harder and more expensive to make, Kloza said, because environmental regulations require mixtures that don’t evaporate as quickly. Fewer refineries can meet those standards, meaning supply is lower, and the raw materials are pricier. (Additives like butane, for example, aren’t allowed.)

Meanwhile, the improving economy and unusually cheap gas — a gallon today is almost 80 cents cheaper than last year — have given a boost to demand. Americans are driving more and burning more fuel, according to the U.S. Energy Information Administration. Demand for gasoline is up 6.6 percent compared to last year.

Still, analysts expect gas prices to gradually fall back toward $2 a gallon, likely by 15 cents a month, once the Indiana refinery is at full capacity and the summer driving season wanes.

“The refinery problem should be a speed bump that delays but doesn’t stop the eventual drop in gas prices,” Green said.