“Look after your tyres and they'll look after you.”
Peer into the bottom of the tread grooves of your tyre and you'll see raised rubber strips. These are the Tyre Wear Indicators; if your tyre tread wears down to their level on any part of the tread, it's definitely time for new tyres, because you're approaching the legal minimum.
Best not to wait until then, though, because the more tread depth you have, the better your tyre will perform in wet conditions. And never let your tread wear down further than the Tread Wear Indicators or you could face a very large fine and other legal punishments. What's more, your car's handling and roadholding will become unstable and unpredictable - not good.
Irregular tread wear also calls for a professional tyre inspection. It doesn't take long to cast your eye over your tyres' treads, and it's time well spent.