Given that a car rolls every mile on rubber tyres, and that I've done a million or so in nearly 50 years of driving, you’d reckon by now I’d have developed a settled view of the above question. Yet I feel less sure than ever that there is a correct answer.
The physical and legal answers are simple. A tyre is mechanically knackered when its case appears through the rubber, and the whole assembly won’t hold air any more. Back home in the Australian bush, this was my dad’s answer. Today, a tyre is legally knackered when there’s less than 1.6mm of tread depth across the central three-quarters of the contact patch. Any thinner and you’ll be nicked, most likely, and certainly fail your MOT.
However, recent experiences of a couple cars - in particular our Range Rover long-termer and a Vauxhall VXR8 I’ve just been driving - remind me that the true threshold of tyre 'knackerdom' needs a much more subjective judgment.
At around 22,000 miles, with an estimated 6000-8000 miles of legal 'meat' left on its tyres, our Range Rover started to feel a mite fidgety. I began to notice a faint rumble through its aluminium structure on coarse surfaces where there hadn’t been any rumble before.
Nobody seemed much bothered about it, probably because it was something you feel primarily at the steering wheel rim, but I have a fetish about refinement in cars and hate to see it decay. I’m quite sure if I had all the money in the world, or even a tenth of it, I’d junk those tyres now (at £300-plus a corner). Not only that, I’d ring up the engineers and find which of the OE tyres was the quietest, and fit those…
The VXR8 came to us over a weekend, for a bit of sport, on 'seasoned' tyres: sidewalls chamfered, treads well legal but also well used. We’d already road tested another car on perfect rubber and pronounced it better in most key respects than equivalent Merc and BMW saloons, so the pressure was well and truly off.
Our kind contact at Vauxhall was merely trying to afford me a bit of extra Easter driving pleasure - and I admit I did set out on a couple of occasions to “do” several deserted roundabouts I know. When a car can slide so beautifully and predictably as a VXR8, it seems a crime to keep it straight all the time.
However, the state of the tyres meant the car didn’t steer as well as the optimal version we’d sampled a few weeks before. Had I owned it, I’d have had to swap the rubber immediately (also at £300 a corner) because this car was no longer doing quite its best work.
So it seems to me that the true answer to the query above is that tyres are knackered when you think they are. At least, that’s true if you want your car to point and track as well as it was originally designed to, which seems to be the point of being the driving enthusiast we all are.