Even better, look for the Wear bars
or, in Europe, Treadwear Indicator
(TWI) bumps in the bottom of the tread
In most modern tyres you'll see little raised lumps in the bottom of the tread grooves. These are often associated with a TWI marking on the shoulder (where the tread meets the sidewall). These are moulded in the rubber and are exaclty 1.6mm high (the legal minimum in most European countries). If the main surface of the tyre is still above the TWI marks across the full width of the tread, then you are probably legal, but if the TWI indicators are level with the rubber surface, even on only part of the tread, then it's time to buy a new tyre.
Some information for you. In Germany, where drivers care about their safety, most drivers change their tyres at or around 3mm tread depth. In the UK where the tyre buyer is after lowest price, many tyres are not changed even at the legal minimum. This is not safe.
If you choose to drive your car with tread depths below about 2mm, then you would be very wise to slow down below 60 mph/100 kph whenever it is raining. If there are deep puddles on the road, stick below 50 mph/80 kph. Aquaplaning becomes very likely above about 90 kph in any depth of standing water. It is almost a certainty above 80 kph (and quite likely below that speed) if the water depth is greater than the tread depth on your tyres.
If you want my advice, change tyres at 3mm tread depth.
Some notes on buying a tyre
Most people go into the tyre store
of what tyre they want. if you are one of the few who knows about tyres, then please ignore the following, but for those who know nothing, these notes may help.
First, not all tyres are the same. However, if you only expect to do low speeds (below say 100 kph/60 mph), then most tyres will do a great job. Tyre manufacture has improved dramatically in the last few years, and tyres made anywhere in the world are probably good enough for this kind of driving style.
As the performance increases, then there are some things you need to know, but first, here's how the tyre dealer sees you.
For better or worse, knowing stuff about tyres is seen as a little sad outside motoring enthusiast circles. You are not a saddo, but this puts you at a disadvantage when entering the tyre store.
The dealer knows that you know nothing about what you are about to buy. You probably don't even have much idea about how much you expect to pay. You have probably heard of a couple of brands, but you are willing to be persuaded that something cheaper is just as good. Industry insiders say that over 70 percent of the buying decision is made by the tyre dealer. That means only 30 percent of customers actually choose their own tyre. The rest do what the tyre dealer tells them.
The dealer makes different profit on different tyres. if a company is having a promotion, then it makes sense for the dealer to persuade you to buy that particular brand. You will believe almost anything he tells you, simply because you have nothing to test his comments against. The dealer also knows that new tyres always make a car feel better, so you are not going to come back and complain if he sells you something not perfectly matched to your needs.
So, that's the background. He knows stuff, and you don't. He's out to make as much profit as possible, without making you feel you've been ripped off.
Here's a brief guide to what you need to know.
First, look at the markings on the sidewall of your existing tyre. It'll say something like 175/60 R 14 75H. You don't really need to know what that lot means, (if you really want to know, then look here) but you need to make sure the new tyre says exactly the same things on the side.
Second, think about how you are going to be driving. Slow speeds and no fast cornering and driving in the dry mean almost any tyre will do. High speed track driving in the rain means you need the best money can buy (that's often a Michelin). In between and you need to get something to suit your style. So for average driving at maximum speeds of say 140 kph, you need a good brand name, but not necessarily a top brand.
Note, in some segments: Off-road, motorcycle, or winter tyres, for example, some of these brands are better than the premium brands.
There are thousands of these brands: mostly made in countries where wages are very low. The following ones are basically good tyres, but do not have the same brand recognition as the ones above, and therefore can be bought for relatively low prices. If you drive relatively slowly (below 80 mph/120 kph)and do not expect to do any hard cornering at speed, then there is nothing wrong with these tyres. You might find their wet grip is less good than a premium tyre, but do not be afraid of buying cheap tyres from these brands.
In addition, there are a number of so-called private brands made exclusively for a named tyre distribution chain, usually by the larger tyre makers. These are also usually good tyres.
I can't think of any more off the top of my head. If anyone knows any more, then let me know and I'll add them.