The best guide I've seen thus far. The guide is done for big truck / fleet owners, but applies in main to personal vehicles also:
KAL TIRE
Tire Wear Conditions
Diagnose Tire Wear
A $5
MILTON Tread Depth Gauge is adequate to most service.
Use of the gauge is shown in this link.
On my pickup truck (sig) I rotate about every 25-30k miles, and use the gauge to take [16] readings per tire annually as rotation is nearly two years apart per service.
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