Arizona law is confusing. There is a state maximum 85 MPH limit, and with that restriction the main speed limit law allows the state to set prima facie speed limits at any speed. But there is a separate law which says that a 55 MPH limit is absolute; driving less than 65 in a 55 zone is "waste of a finite resource" and driving 65 or more is speeding. There is another law which sets absolute speed limits of 65 and 75 on rural Interstates. In fact the rural Interstate speed limit is 75 (and a court has accepted a challenge to a ticket on the grounds that traffic engineering standards call for a higher limit) and the limit on most other rural roads is 65.