So I went outside and looked at the tires on four vehicles. Every one of them has slight "cracks" in the tread grooves similar to what Sam showed (maybe not as large though), except for some new tires that are about 4 months old.
The tire codes date the various tires from 2007 to 2016. The 2007 LT tires have the least amount of visible cracking. None of the tires have over 35k on them.
From that, one might conclude that tires show tread groove "cracking" without it necessarily being a sign of old age. What do you think?
