When Jay Ricker, owner of the BP gas station off Interstate 70 in Plainfield, Ind., set the price of unleaded gasoline at $3.44 per gallon on Monday of last week, it was 4 cents higher than the Friday before.

That alone might have been irritating to drivers paying the highest gas prices in more than two years. It was even more so because it happened on a day when the price of crude oil, which is used to make gasoline, fell almost $1 a barrel.

“It’s up 20 cents one day, down 10 cents the next day,” said Oscar Elmore, a courier who was filling up his Ford Taurus at a RaceTrac service station in Dallas recently. “It sounds kinda fishy to me.”

Gas prices rise when oil prices rise, and fall when oil prices fall — except when they don’t. What you pay at your gas station depends on an array of factors, from what happens on an exchange in New York to what the competition is charging.

This can rankle drivers, especially these days. Gas reached a national average of $3.51 a gallon on Monday. That’s up 14 cents, or 4 percent, over the past week. The week before, the average rose 20 cents, the steepest increase since September 2008.

A year ago, the price was $2.75. The average is the highest it’s ever been this time of year, and analysts expect it to climb higher. Unlike an iPhone or a pair of jeans or a Big Mac, oil and gas are commodities, and their prices can change every second at the New York Mercantile Exchange and other trading hubs. Those far-off changes affect the cost of the next day’s commute.

Sellers of commodities, like gas station owners and refineries, price their product based not on what it costs to produce it, but on what it costs to replace it. Stations like the Plainfield BP, which gets shipments of gas several times a week, must constantly adjust their prices to keep up with the changing costs.

Every day at 5 p.m., BP tells Ricker what the rack price will be starting at 6 p.m. That price is good for 24 hours. Ricker hires a trucker to go to the terminal a short drive away in Indianapolis, fill ‘er up with 10,000 gallons and bring it to his station. Then Ricker decides what price to charge customers based on his ultimate concerns: the stations that share an intersection with him.

There are only two or three pennies per gallon in profit selling gas for most station owners. What Ricker really wants is to attract customers to sell the truly precious liquids: Not the gasoline and diesel outside, but the water and soft drinks inside.

Three times a day, his station manager, Debbie Sennett, records his competitors’ prices. When the competition lowered prices on Tuesday, so did Ricker, to $3.24 per gallon.

“Gasoline is the only product in this country that if you’re a penny different people will go out of their way to go somewhere else,” Ricker says.

Wholesale gasoline prices have risen 38 cents per gallon, or 15 percent, since the first uprising in Libya on Feb. 15. When wholesale gas prices rise fast, filling station owners get squeezed or even lose money because competition prevents them from raising retail prices as fast as costs are rising.

So if it seems that station owners take their time lowering prices when oil and wholesale gas get cheaper, it’s because that’s exactly what they do.