Why the narrower tires for winter? Wider tires seem like they would be better for the very wet winters we have here when traction is needed more. Our summers are typically quite dry and I would expect a narrower tire to work better then.
I'm right at 35k miles now and have 4mm tread depth remaining. The listings for these tires have them guaranteed to 60k miles (but I'm not eligible for any warranty on them because I bought the truck used)
Intuitively you are right - this is what I would have thought as well. There are conflicting opinions, but while "the wider the surface, the better the grip is logical", wider tyres are apparently not as safe on slush or water covered roads in winter. They are also not as good on fresh powdery snow. The logic here is that the narrower tyre cuts easier into slush/water/fresh snow in order to reach a surface where it can grip.
When driven in such conditions, tyres tend to build a wedge (while displacing material) at the front and the wider they are, the larger the wedge.
There were tests done by ADAC (Allgemeiner Deutscher Automobil-Club / General German Automobile Club) - a well-respected independent organisation (I guess the US equivalent is AAA), which found that winter tyres should be narrower. Winterreifen-Test: Schlankere Reifen sind überlegen - SPIEGEL ONLINE
The major conclusion is that in critical situations narrower tyres perform better. Drivers should install the narrowest manufacturer-approved tyres for the winter. This seems to be the prevailing opinion and this is what I go by as well.
At the same time Continental recommends to put the widest fitting tyres for the winter for better grip. Wide tires
Go figure, but I still like the narrower and ADAC's logic for winter. Also if car manufacturers recommend narrower for winter, there has to be a reason.
60k miles is a lot of miles - almost 100k km! I never expect a tyre to do this for me and I am not an agressive driver by any stretch of the imagination.
I see Corporal has done 95k miles and this for me is out of this world, simply extraordinary.
35k miles for passenger-style tyres is not little to me, but this is just me. Here we are happy at 60k km and unhappy below 40-50k depending on the cost of the tyre (i.e. I would be pissed off if Michelins lasted less than 50k for me, but would be OK for Kumho - also because I'd be getting rid of them
). This is a broad generalisation of course.
Continental recommends that tyres be replaced at 4mm for winter tyres and 3mm for summer tyres. Tyre Knowledge - Remaining Tread Depth
I suppose you also should be able to see how much you have left until you reach the thread wear indicators.