California Code, Vehicle Code - VEH § 660

The “unladen weight” of a vehicle is the weight equipped and ready for operation on the road including the body, fenders, oil in motor, radiator full of water, with five gallons of gasoline or equivalent weight of other motor fuel;  also equipment required by law, and unless exempted under Section 66l,  1 any special cabinets, boxes or body parts permanently attached to the vehicle, and any machinery, equipment or attachment which is attendant to the efficient operation of the body or vehicle.  Unladen weight shall not include any load or any machinery or mechanical apparatus, such as, but not limited to, wood saws, well-drilling machines, spray apparatus, tow truck cranes, and grinding equipment.  The unladen weight of a vehicle shall have no application in determining any fee under this code or the Revenue and Taxation Code other than Section 9400.