Cal. Rev. & Tax. Code § 8801

If any user fails to make a return, the board shall make an estimate of the amount of fuel used by the user which is subject to the tax. The estimate shall be made for the month or months in respect to which the user failed to make a return and shall be based upon any information available to the board. Upon the basis of this estimate the board shall compute and determine the amount required to be paid to the State, adding to the sum thus fixed a penalty equal to 10 percent thereof. One or more determinations may be made for one or for more than one month.