Section 27450 Of Article 4. Tires From California Vehicle Code >> Division 12. >> Chapter 5. >> Article 4.

. When any vehicle is equipped with any solid tire, the solid tire shall have a minimum thickness of resilient rubber as follows:
  (a) If the width of the tire is three inches but less than six inches, one inch thick.
  (b) If the width of the tire is six inches but not more than nine inches, 1 1/4 inches thick.
  (c) If the width of the tire is more than nine inches, 1 1/2 inches thick.