Cal. Business and Professions Code - BPC § 13800. Section 13800.
(a)
Notwithstanding any other provision of this division, a rental vehicle’s fuel gauge installed by the vehicle’s manufacturer may be used in a rental transaction by a rental company to calculate an optional charge for fueling when any of the following occurs:
(1)
The customer could have avoided incurring the charge by returning the rental vehicle with the same amount of fuel as was in the fuel tank at the commencement of the rental.
(2)
The customer chose to purchase the amount of fuel inside the fuel tank at the commencement of the rental.
(b)
Nothing in this section shall be interpreted to preclude a rental company from offering additional fueling options to customers besides those described in subdivision (a).