The depth of tread remaining doesn't really affect tyre grip in the dry as long as the profile is not worn at the shoulders. However, the ability to shift water out from between the tyre and the road on a wet road diminishes as a function of the depth of tread remaining.

I don't run tyres down to the minimum 1.6 mm as approaching 2 mm aquaplaning is far too easy to achieve on standing water so changing at 3 mm like emergency services are supposed to do is a good idea.

But I did look up the MOT history of a UK police transit that appeared in a newspaper article recently and found that it failed it's last MOT on front nearside tyre with canvas showing through amongst other things!

Back in the 1960s (when there was no legal minimum tread depth requirement, just 'no canvas showing' ) I followed a friend and saw his car perform a 360 degree spin when he braked on a wet road. He had had 4 smooth tyres, bought 2 new one but put one on left front the other on right rear.

I agree with SLO76. Swap the front to the rear so they get to sub 2.0mm about the same time so you can change all four at once. You should get another 10K out of them.

I wouldn’t change tyres until they’re on or very close to the legal limit unless I’m feeling a noticeable drop in grip levels. Cost is a factor but also environmental issues too. If everyone suddenly started changing tyres so early the amount of extra resources required nationally would be huge never mind globally. Stick with a good medium to premium tyre and there should be very little grip loss until they drop well below 2mm.

Not sure how you work that one out, ie environment issues, they end up as dust on the road anyway uncontrolled, at least they get turned into other things when recycled,

and as for road grip at legal limit, fair comment in the dry, but not in the wet 2mm or less cannot clear as much water as 3mm can unless they have found a way of dispersing water much quicker and in greater volume

I see it as a shameful wast to throw tyres away that still have thousands of miles left in them

Funny you should say that as I used to take tyres to minimum legal limit, but then I kept getting punctures that were not legal to repair because the law said a puncture in a certain place had to be scrapped which meant buying a new tyre.

and I have had more than my fair share of scrapable punctures due to builders dropping darn great roof nails in the road with other people dropping sharp objects on the road that for some reason seemed to be aimed at my tyres

apart from that I do know how tyres work and understand whats involved in making them, which they are still going to be made regardless of whether I change a tyre at 1.6mm or 3 and Im contributing to rebuilding roads and other materials that tyres are being used for to save the earths resources, But then you knew that anyway;)

Personally I do not have a set rule about when to buy new tyres other than I would never drive into October with tyres showing under 2.5mm. By the time you hit January or February you will be down to the legal limit and buying new ones so why not get some a few months early and have the benefit if a safer car during the wet and slippery Autumn months.

I will be changing to all season tyres, probably GoodYear as noticeably cheaper than Michelin Cross Climates and just as good it seems.

Have a look to see if Kleber Quadraxers are available in your size. Had them on 3 cars and they are magic. Excellent grip in the snow and quiet on dry summer roads. In the common 205 55 16 V size they are about £20 a tyre less than Michelin and £5 a tyre less than Goodyear. Take a look at the tread pattern virtually identical to the Cross Climate, hardly surprising since the are made by Michelin. If I were buying a set of tyres today and they were avaiable in my size I would not hesitate.

When I was racing we used to get new road tyres shaved before using them. As a novice I thought this was crazy, buy a new tyre and then pay to get it worn was surely madness. But the extra grip in the dry using a tyre with about 3.5mm compared to the same new tyre with full tread depth was amazing (even when the releasing compound had been worn away). It was far more stable (braking and cornering) since the tread blocks were not moving about and it had far less tendency to scrub the shoulders off than a tyre with full tread, in some cases they would actually last longer. With 3.5mm the tyres were fine in the wet but once down to 3mm it was time to get a new set (or an extra set of rims with suitable tyres), in the dry they were fine right down to 1.6mm. On a streaming wet track you would be nowhere other than doing an impersonation of a mobile chicane.

Track specific (but still road legal) tyres are normally made with about 4mm of tread but not all classes allow them.