Having worked on a farm for many years I have unloaded thousands of used tyres supplied by numerous tyre fitters, both national companies and small local companies. The tyres are used on a farm to weigh down plastic sheeting used to seal silage clamps. The one thing that has always been noticeable is the large number of apparently 'perfect' tyres in every consignment, the only 'damage' being a large and conspicuous cut, clearly caused by an angle grinder, in the sidewall of every tyre. The explanation became clear when a friend clouted a kerb with both nearside wheels and went to the tyre supplier to have his tyres checked, he was advised to have both changed. If you think about it why would they advise otherwise in this litigious world we live in. At least by changing the tyres they make money rather than increase their insurance cost.