You surely have noticed that gas prices have been creeping up nationally in the last few weeks. In suburban Michigan, the price of regular has climbed ten cents in about a week. In California, gas prices are averaging about $4.

Nationwide average gasoline prices are now 30 cents higher than they were five weeks ago. They are now at the midway mark between this year's high price of $3.94 a gallon – hit April 5 – and the recent low of $3.33 hit just over five weeks ago, according to AAA.

As gas prices climb, look for more fuel-efficient vehicles to increase in demand, and thus probably prices. Getting a jump on fuel-thrifty vehicles in August is probably a good idea before prices reset in September.

Here are the factors impacting prices at the pump:

– A fire at a Chevron crude-oil refinery has impacted supply on the west coast, pushing up prices there. The average price of regular gas in California climbed from $3.86 to $3.94 in less than a week, according to The Richmond refinery produces 16 percent of the region's daily gasoline supply. The fire knocked out a unit that makes a specialized blend of cleaner burning gasoline that satisfies air quality laws in California, Oregon and Washington.

– Before the California refinery problem, a series of refining and transportation problems in the Midwest were impacting prices, according to tracking by the Oil Information Service. Last month, said the OIB, an Enbridge pipeline was shut down in the Midwest after a . July also saw problems at a BP refinery outside Chicago, and at a Marathon refinery in the region.

– Oil prices are rising globally. Crude oil prices have been climbing of late, jumping nearly 20 percent in less than six weeks. Traders have blamed an array of factors on the run-up. Unrest in Iran, and continued revolution in Syria. Syria is a tiny oil producer, but the worry is the unrest there could spill into in Iran and Saudi Arabia and impact supply through the Persian Gulf.

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