The stupid things last forever. When I bought my truck, I put on 265/75/16 A/Ts as I was unsure about going to the 285/75/16. That was 3 years and 40k miles ago. I really want to go 33" tires, but a quick check with the tire gauge reveals that they still have 11/32" of tread left. They started with 16/32". They are going to dry rot before they wear out.
