At a California State Senate hearing on February gasoline price spikes, Consumer Watchdog issued a report that said California oil refiners have about half as much gasoline on hand as the rest of the US, creating a volatile market that leads to gasoline price spikes.

The report, “Price Spiked: How Oil Refiners Gouge Californians On Their Gasoline and What It Costs,” showed that Californians paid $550 million extra for their gasoline in February compared to drivers nationwide by calculating the difference between US and state prices and allotting for consumption.

On Monday, March 23, 2015, Californians paid 84 cents more per gallon than the rest of the nation for their gasoline, the group said in a statement.

“Oil refiners are gouging Californians by keeping the state running on empty so that when refineries go down gasoline prices shoot up,” said Jamie Court, president of Consumer Watchdog. “It’s time for the legislature to require that oil refiners keep another week of gasoline inventory on hand to avoid perennial price spikes in the third largest gasoline consuming economy in the world.”

Read the report here.