California Code, Revenue and Taxation Code - RTC § 60116

(a) Commencing January 1, 1998, and on each January 1 up to and including January 1, 2013, the board shall establish a tax rate per gallon, rounded to the nearest one-tenth of a cent ($0.001), by multiplying the average retail price per gallon (including the federal excise tax and excluding the state excise tax and the sales and use tax) of diesel fuel sold in this state by a percentage equal to the combined state and local sales tax rate established by Part 1 (commencing with Section 6001) and Part 1.5 (commencing with Section 7200) of Division 2 of the Revenue and Taxation Code and Section 35 of Article XIII of the California Constitution. The average retail price per gallon shall be the average of weekly retail prices for the 12-month period ending August 31 of the year prior to the effective date of the new rate. In determining the average retail price per gallon, the board shall use the weekly average retail price published by the State Energy Resources Conservation and Development Commission, in its publication “Fuel Price And Supply Update.” In the event the “Fuel Price And Supply Update” is delayed or discontinued, the board may base its determination on other sources of the average retail price of diesel fuel. The board shall make its determination of the rate no later than October 1 of the year prior to the effective date of the new rate.

(b) The tax rate established by the board on January 1, 2013, shall remain in effect only through June 30, 2013.

(c) Commencing on July 1, 2013, and on each July 1 thereafter, the board shall establish a tax rate per gallon, rounded to the nearest one-tenth of one cent ($0.001), by multiplying the average retail price per gallon (including the federal excise tax and excluding the state excise tax and the sales and use tax) of diesel fuel sold in this state by a percentage equal to the combined state and local sales tax rate established by Part 1 (commencing with Section 6001) and Part 1.5 (commencing with Section 7200) of Division 2 of the Revenue and Taxation Code and Section 35 of Article XIII of the California Constitution. The average retail price per gallon shall be the average of weekly retail prices for the 12-month period ending on the last day of January prior to the effective date of the new rate. In determining the average retail price per gallon, the board shall use the weekly average retail price as reported by the United States Energy Information Administration (EIA). In the event the EIA information is delayed or discontinued, the board may base its determination on other sources of the average retail price of diesel fuel. The board shall make its determination of the rate no later than March 1 of the same year as the effective date of the new rate.