I use 2 criteria for tyre life.
Tread depth. 3mm is a "get reacy to change" warning.
Age. 6 years max.
I rotate all 5 on the Mog so the spare gets use instead of just sitting there getting old and change all 5. It isn't there just for looks and needs to be usable.
When replacing I prefer tyres with good wet performance as that is when it is most needed. I currently have Avon ZV7 on recommendation here in TM as I couldn't get Yokohamas that were on before that. Well I could but in a lower profile....I don't want reduced ground clearance! Avons are performing well.
