I know exactly - 50,000 kms or six years. I've just changed them. Tyres are polluting, ok, but irrelevant for the most part in dealing with global warming - though a tonne of tyres is responsible for around 700kg CO2 emission. There are about 30 kg of tyres on a medium car. So one tonne represents about 30 changes of tyres or 1.5 million miles, A petrol car will produce around around 4.6 tonnes CO2 per year, say 12,000 miles. So 1.5 million miles will produce 500 tonnes of CO2 or more, conservatively. Makes the tyre CO2 rather pale in comparison. And the petrol and diesel emissions are likely just as detrimental to health. So don't go on about tyres please - a red herring.