Of course tyres do not stop working at any given temperature that we are likely to encounter... As per Steve Henning's comments... they may tail off in performance outside the design temperature range, but that's what happens in real life and tyres have been designed to cope with. By far a bigger issue is what if you have a crash with "proper" winter tyres on and the speed rating is not what is recommended by the manufacturer ! Ideal wriggle-out possibilty for them, all because we were "trying to be smart" and fit a "better" tyre .... What happens if we have winter tyres on and the sun comes out - do we change tyres and then back again at night ? Here in the UK, we are having -6 deg C at night and + 12 deg C in the day...not a great temperature range and our "all weather tyres" seem to cope admirably... Nick