[prndl]

The "wear bar"is the proper indicator for replacement.

See the 'bar' in between the tread?

My tires are wearing at the outer edge because I ran them at too low a pressure.
The circumference of the outer edge is smaller than the middle of the tread. If you run the tire too low as the tire turns one revolution the outer edge must scuff to keep up with the load bearing inside tread.