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I recently read from a reputable source that tyres should be replaced at 3.2mm of tread thickness, not 1.6mm as the previous dogma suggested. My tyres are all in the 2.0mm - 2.5mm range.

I understand that deeper tread is beneficial in the wet, but the fine article states shorter stopping distances for deeper tread in the dry as well. That seems counter intuitive to me, I thought that if there were no rain then tyres wouldn't have grooves on them at all! How could a deeper tread shorten stopping distances in the dry?

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I don't believe that the tread depth actually is the deciding factor. By the time you wear the tires down that far they've had a LOT of heat cycles put on them. As a tire gets heat cycled repeatedly, it gets harder and looses grip. That's what I understand to be the real issue in those tests.

As far as the depth to replace at, that's been debated quite a bit. Tire shops in many areas are saying 4/32" for tires that never see snow and 6/32" for tires that do get used in the snow. Some states in the USA have bumped standards up to 3/32" as well.

On the other side, most states are still at the 2/32" wearbars as the standard. Environmental and conservation concerns are pushing others to go with the 2/32" and "don't be stupid" when driving in the rain.

The main risk of insufficient tread depth in rain is hydroplaning at speed. If you're not going fast enough to hydroplane, there's no real benefit to throwing out tires early. If you only drive slowly in occasional light snow, same idea.

Best option would be to replace tires daily, but that's just silly. :-) So, you have to pick the appropriate cost/risk value that fits within legal limits.

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I disagree about replacing daily, as tires do have a break-in period. But the rest of your comment seems very logical, thanks. I did not consider the annealing effect on the rubber might be the issue, and not specifically the tread depth. –  dotancohen Mar 22 '12 at 18:51
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It depends on where you live and the climate and season. In Sweden for example you must, according to the law, have at least have 1,6 mm in the summer and 3,0 mm in the winter, or winter conditions. And that sounds reasonable to me.

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