I said gut because thats what I do. I watch them and look for signs. Uneven wear, tread dropping below what i feel is safe, cracks in sidewalls or dry rot. If I start to see signs, I replace them. I also feel especially for overland that one shouldn't piece meal it. When one gets down or cracked (assuming it isn't new with a flaw or damage), I replace all 4. YMMV, we all have different methods!