California Code, Revenue and Taxation Code - RTC § 8651

(a) An excise tax is hereby imposed for the use of fuel at the following rate per gallon:

(b) If the federal fuel tax is reduced below the rate of fifteen cents ($0.15) per gallon and federal financial allocations to this state for highway and exclusive public mass transit guideway purposes are reduced or eliminated correspondingly, the tax rate imposed by this section, on and after the date of the reduction, shall be increased by an amount so that the combined state and federal tax rate per gallon equals the following:

(c) If any person or entity is exempt or partially exempt from the federal fuel tax at the time of a reduction, the person or entity shall continue to be exempt under this section.