I have always got exceptionally good mileage out of my all season tyres. You'd imagine that their more 'aggressive' tread pattern would increase noise and reduce mileage but this is not the case.
Its normal in the tyre trade to recommend replacing tyres at 3mm,Even summer tyres, ,especially if you want good grip in snow. Yet Michelin claim their Cross climates give good grip in snow right down to the legal limit of 1.6mm. (although some countries may apply a 3 or 4mm legal minimum limit in winter) This may be a reason to chose cross climates despite their cost. But I'd still rather have 3mm despite what Michelins marketing people say .