In California, gasoline refinery production is not keeping pace with demand, which causes the state to rely on more imported gasoline. California currently imports 10 percent of its gasoline, and those imports are expected to grow to 20 percent by 2010. Over the past several years, gasoline demand has grown between two to three percent per year primarily as a result of our state’s growing population. However, consumers are also choosing less fuel-efficient cars. Of the new vehicles purchased in 2003, one-half were minivans, SUVs or light duty trucks. The supply/demand imbalance becomes even greater in the summer months when gasoline consumption typically rises by nearly 10 percent during the “summer driving season”.

If a refinery problem occurs in California, the state relies on tankers to ship gasoline to California from the half dozen refineries around the world that can produce the state’s clean-burning gasoline. Even the closest refinery, in the state of Washington, is 7 to 10 days away, and gasoline from Texas must come across Panama. In other states, there is a system of pipelines that move gasoline and diesel products throughout the mid-West and eastern states, especially if there is a refinery problem.

Gasoline consumption typically rises during the summer because more people are on the road during the “summer driving season”. For example, in 2002 and 2003, daily gasoline demand rose 8 percent and 9 percent, respectively, when you compare consumption in January to August. […]

High world crude oil prices have contributed to raising the price of all petroleum products. However, in the most recent California gasoline price sudden rise, oil prices are secondary to lost refinery production and resulting product shortages.

Extracted from:

http://www.energy.ca.gov/gasoline/gasoline_q-and-a.html