I have never gotten the mileage rating out of a tire(s). Typically I need to replace tires when they just don't perform in foul weather any longer which could be between 3/32nds & 4/32nds(and some tires, even 5/32nds).
Foul weather doesn't just mean SNOW/SLUSH, it could mean that nasty rainstorm when the wipers are on HIGH SPEED and it's nighttime.
And too, the tires may only have ~35K-40K miles on them when they're rated for much higher(60K-70K). I suppose that if I lived in a climate where it never snowed, I could push them further toward that 2/32nd of tread life however, even when I run winter tires separately, those A/S tires that I'm speaking of just don't feel right anymore when the tread is getting past the half way point.
Oh sure, even when tires get worn past my comfort zone, said tires perform just fine on a beautiful sunny day in JUNE. But when it rains and the wind picks up and you're on the hiwy and it's DARK outside, I want better for me & my family.
Yeah, there's more to it than just the above^^^ but, you get my meaning? And I'm sticking to it!