A.   The speed limits on U.S. Highway 80 through the corporate limits, which include that portion of Fremont Street east of Sumner Street and that portion of Sumner Street north of Fremont Street, shall be established and posted by the Arizona highway department.
   B.   The speed limit on all other thoroughfares within the corporate limits shall be twenty five (25) miles per hour, unless posted otherwise. (1972 Code)