Gas prices generally increase as refineries start to change winter-season gas to more expensive summer-season gas, lowering production. - 

If you filled up your car at a gas station this weekend, you probably noticed that it cost you a bit more than it did the last time you were at the pump. The average price for a gallon of gas is about 13 cents higher than it was two weeks ago. Several factors contributed to that rise.

One of those factors -- refineries are transitioning from producing cheaper winter-blend to more expensive summer-blend gasoline. Refineries also use this time of year to do maintenance on their facilities, which means they produce less gas, lowering supply.

"Well, basically, they are cleaning their machines and making sure everything is functioning properly ahead of the high demand season," says Gregory Dacko, a senior economist at IHS. Yet another reason for the higher price at the pump, says Dacko, is cutbacks in production by OPEC.

But the single biggest factor is the price of crude oil. "The price of crude oil makes up about 66 percent of the price of a gallon of gasoline" says Avery Ash, a spokesperson for AAA.

Crude oil is up by 14 percent since mid-December. Analysts don't expect prices to rise quite as fast as they did this time last year. But last Friday did mark the first time in 2013 that the national average price of a gallon of gas was higher than that same day in 2012.

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