Cal. Rev. & Tax. Code § 8654

No tax shall be imposed as to any fuel used in any motor vehicle owned by any county, city and county, city, district, or other political subdivision or public agency when operated by it over any highway constructed and maintained by the United States or any department or agency thereof within a military reservation in this State. If any such motor vehicle is operated both over such highway and over a public highway outside the military reservation in a continuous trip the tax shall be imposed only as to that portion of the fuel used to operate the vehicle over the public highway outside the military reservation.

Nothing contained in this section shall be construed as an exemption from the tax of the use of fuel in any motor vehicle operated upon a public highway within a military reservation, which highway is constructed and or maintained by this State or any political subdivision thereof.

As used in this section, “military reservation” includes any establishment of the United States Government or any agency thereof used by the armed forces of the United States for military, air, or naval operations, including research projects.