You might want to fill up the gas tank this weekend. At the moment, the state gas tax is 36 cents per gallon in California. That changes on Monday, July 1st. That's when the tax goes up by about 10 percent to nearly 40 cents per gallon. The increase approved earlier this year by the Board of Equalization. When you combine the state tax with the federal gas tax you'll end up paying 72 cents extra for every gallon you buy -- the most expensive gas tax combination of any state.