The road is paved a few miles out of Chino Valley to about the national forest boundary, but then is all gravel. I wouldn't suggest doing this in a rental car. It does go over some hills and can be quite rough. Additionally, there are some low water crossings with at least one major wash, and August can often see some pretty serious rain. It's also not particularly well-signed, with several branching forest roads.
But, for a forest road in Arizona, it's not bad.
Use Hwy 89 from Ash Fork. It is somewhat scenic. It is a bad idea for people not familiar with the area to take "back roads." And don't depend on GPS. Have a map. Google will show you all kinds of routes that you should not take.
The Williamson Valley road is indeed not a fast route, but there are positives. We have taken it once, and the views of the big ranches are spectacular. The Sunday we drove it there was a family sitting on a fence watching another person ride a horse. It looked like a Norman Rockwell painting!
thanks for the info, but can you tell me if the road is all unpaved or there are miles paved ? thank you
Expect more gravel and rock than pavement.
Read Colin's post above. IMO it appears to be an inadvisable route, especially in a rental car. Traveling on such roads in a rental car can void your contract, and any damage under such circumstances can lead to full value of the vehicle being charged to your credit card.
We took this road in May and still remember it as one of our favorite afternoon drives. The road is gravel, but it was an easy, beautiful drive, as long as there's no recent rain. It was very safe, easy to drive in a rental car and well worth the extra time. Do it!