So two to three years ago? They are still comparative youngsters - Properly stored tyres stored in conditions of humidity, heat etc where they suffer minimum degradation are allowed to be sold as "new" until they are 5 years old I seem to remember (may have changed since I made a living at it though) One big factor worth always remembering about tyres though is that the rubber oxidizes and heat and sun etc exacerbate this so the conditions in which it's been stored have a lot to do with it if the tyre is not fresh out of the factory. A brand new tyre needs to "break in" by not subjecting it to extremes of use for about 100 miles or so. This allows the casing in particular to become more supple and the tread surface to "work harden" to a certain extent (it's much more complicated than this and I'm not a chemist or tyre designer but we were always taught to advise customers to drive with care and not subject the tyre to violent braking, cornering or acceleration for the first 100 miles or so (unless there's no option of course). By the way, just while we're on the subject of advice, I've was involved in performance tyre testing back in the 1970's - endurance, grip, etc etc. and found that tyres with less than 2mm of tread depth suffered a considerable reduction in grip on wet road surfaces compared to identical tyres with more depth of tread. It's all due to the tyre's ability to displace the water and allow the rubber to get through to the road surface. Of course things vary in degree between makes of tyre and car. Weight of the vehicle, age and width of the tyre and how much water is on the road surface all introduce variable factors but once you drop below the 2mm things become much more dodgy in the wet! It's noticeable how much more sharply the graph line dipped at that point.
When I bought my EVO i was caught off guard going around a corner in the rain. It momentarily lost grip, it was very unnerving, and upon inspection the tyre that had lost traction had less than 2mm of tread. I got that changed sharpish!
I have a tyre on my EVO that has cracking around the rim, i've been thinking about a switch to the spare tyre. That one doesn't have any cracking but it has only about 3mm of tread, so I've been slacking on it. Should I switch that ?As your tyres age keep an eagle eye out for sidewall cracks and cracks around the bead area. Also look into the tread grooves for cracking at the base of the grooves. Because the surface will oxidize much more quickly mild cracking which is surface only, especially on sidewalls, can probably be lived with but if the cracking anywhere exposes part of the internal cords of the tyre it needs to be scrapped immediately - in fact it's an offence and the boys in blue will be very unhappy about it. Oh, and remember that tyres can, and often do, degrade or suffer damage on the inner sidewall where you can't easily see it. So every now and again take a good look at the inner sidewalls of your tyres. This is especially good advice if the vehicle is often driven on vary poor surfaces or frequently mounts the curb.
The choice is 5mm tread with cracking on the rim, or 3mm tread and no cracking.