Once any part of the tyre gets to around 2mm I'd replace both of the tyres on that axel.
As they're runflats and cost £200+ each, I would hope not to have to replace 4 at once.
Dumb question but do tyres "go off"? I don't do many miles in my car and I can't even remember how far back I changed them (has to be years, possibly 2-3 or more).
The fronts are probably around 2-3mm depth now but close inspection of the rubber shows cracks, etc and quite frankly they seem of late to be particularly crappy grip wise.
[TW]Fox;17449935 said:You will do eventually though - ie with me I found that on my 3rd set of rears I needed a new set of fronts at the same time.