California Code, Vehicle Code - VEH § 4000.4

(a) Except as provided in Sections 6700, 6702, and 6703, any vehicle which is registered to a nonresident owner, and which is based in California or primarily used on California highways, shall be registered in California.

(b) For purposes of this section, a vehicle is deemed to be primarily or regularly used on the highways of this state if the vehicle is located or operated in this state for a greater amount of time than it is located or operated in any other individual state during the registration period in question.