I assume there is not actually a Hwy 666 but instead part of Hwy 191 is referred as "Devil's Hwy", maybe between Morenci and Hannagan Meadows (or Alpine)?
If you are a Mormon or a Catholic it is Highway 191, if you are a motorcyclist or sports car driver it is '666'.
https://azride.com/devils-highway-run/2557977
Arizona Churches banded together many years ago to have 666 designation removed and they were successful, mention it to a Biker and it's '666'!
Distance: 265 Miles
Elevation variation <6,000 Ft.
Number of curves < 1,000 The Rt. 666 Run
starts/ends in Springerville, AZ with the southernmost point at Three Way, AZ. The ride loops
clockwise, so we’ll go downhill through the broad
sweepers of NM Rt 180 and we’ll enjoy the
switchbacks of AZ Rt 191 going uphill:
- Rt. 191 South (AZ) Alpine
- Rt. 180 (NM) Luna, Alma
- Rt. 78 (NM, AZ) Mule Creek, Three Way
- Rt. 191 North (AZ) Clifton, Morenci, Hannagan
Meadow, Alpine, Springerville