A little insight, Back in the day when I attended automotive school and front end alignment shop. The instructor discussed tire wear. Showed us a tire with a wavy thread wear which was due to bad or blow out shocks. If you every seen a vehicle on the road and the it has a blown out shock the wheel just bounces up and down and loses contact with the road. Its the weirdest thing and pretty scary when you see it.
Another example isI use to get this floaty feeling in my mustang. it had Koni's in the back with 40k miles on them, and when you did the bounce test it seemed normal, change out the shocks and the floaty feeling was gone. That floaty feeling was due to the wear point in the shock tube.
Now in your case, the shocks are at 60k, and with crappy roads like we have here in NYC. they have taken a beating. They haven't blown out but they have worn a sweet spot in the tube. The wear indication at the outer edge of the thread is from the camber dialed in to the car and the wheel oscillation due to the shock wear point in the tube.
By the way I have a 370z and after 30k I trashed the factory shocks and installed a Nismo S-tune suspension. The handling was like night and day. Just the new shocks alone will make the car handle better, and if you are going to stick with factory, I would upgrade to the shocks that come on the Nismo, they are made by Tocki which is a well know performance shock manufacture.
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