California Code, Vehicle Code - VEH § 24017

(a) A commercial motor vehicle, as defined in Section 260, operated by a motor carrier, whether the motor carrier is a private company or a public agency shall be equipped with a speedometer that shall be maintained in good working order.  The speedometer shall indicate the vehicle's speed in miles per hour or kilometers (km) per hour and shall be accurate to within plus or minus 5 miles per hour (8 km/hour) at a speed of 50 miles per hour (80 km/hour).

(b) This section does not apply to an autonomous vehicle, as defined in Section 38750, that is not capable of operation by a human driver seated in the vehicle, provided that this exemption is consistent with, or authorized by, any applicable federal law, regulation, or exemption thereto.