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.