US64 west turns into US160 in AZ. US160 is two lane with a few rough spots. Take care traveling through the Navajo reservation, the locals have there own style of driving... US160 will end on US89 which will take you south to I-40 in Flagstaff and west into the heat.
There is no way around the hottest part, around the Colorado River, when heading for San Diego, I recommend taking I-40 all the way from Amarillo , just plan of driving from a cool spot like Williams or Seligman, AZ all the way to San Diego so you don't have to stop in the heat.
Make sure you cooling system is up to the task.
If you want to see southern NM and AZ, do it in the winter/spring months.
