California Code, Revenue and Taxation Code - RTC § 8651.7

(a) The owner or operator, except an interstate user, of a vehicle propelled by a system using liquefied petroleum gas, dimethyl ether-liquefied petroleum gas fuel blend, liquid natural gas, compressed natural gas, or dimethyl ether may pay the fuel tax for the use of those fuels by paying an annual flat rate fuel tax according to the following schedule:

(b) The annual flat rate fuel tax described in subdivision (a) shall be an annual tax. The annual period shall be that period from the end of the month in which the tax was paid to the end of the month prior in the following calendar year. When an owner or operator elects to pay the annual flat rate fuel tax on more than one vehicle, the owner or operator may request that the department prorate the tax due on a vehicle added during the annual period, so that all vehicles have the same annual period. In the year a vehicle is added, the annual flat rate fuel tax for that vehicle shall be calculated by dividing the fee set forth in subdivision (a) by 12 and multiplying the resulting amount by the number of months remaining before the beginning of the next annual period.

(c) The department shall adopt an identification procedure for vehicles with respect to which the annual flat rate tax described in subdivision (a) of this section has been paid.

(d) Subdivision (a) shall not apply to dimethyl ether-liquefied petroleum gas fuel blend or dimethyl ether until July 1, 2021.