Cal. Rev. & Tax. Code § 8502

The commission may impose, in addition to any other tax authorized by this division, a tax on the privilege of selling within the region, motor vehicle fuel, as defined by Section 7326. The tax shall not apply to motor vehicle fuel used to power aircraft. The tax shall be levied at a rate established by the commission, but not exceeding ten cents ($0.10) per gallon. Commencing on January 1 of the year following the election approving the tax, the tax may be imposed for a period not to exceed 20 years.