Tires are the cornerstone of the vehicle you drive. It does not matter whether you own the engine of the century, a mind-blowing suspension system, or the car with the most advanced aerodynamics on the market; if your very contact with the road is not reliable, it is both your driving experience and your safety that you are compromising. Hence the importance of knowing when the time to change your car tires has come.

Beware of the law

Developed countries usually have some kind of law or a set of rules regulating tire use and attrition. For instance, the province of Quebec in Canada makes the use of winter tires compulsory between December 15 and March 15, and fines as high as $300 are given to offenders. Another example is the limit many U.S. states impose on the wearing down of tires: tread depth must be over 2/32” for a tire to be considered legal.

While laws and rules such as these may help drivers know when changing their car tires is a pressing need, they nonetheless do not cover specific climatic circumstances and road conditions under which legal tires just don’t suffice. Tires need to be safe anytime and anywhere, period. Therefore, it is both useful to know what level of attrition is still acceptable in specific situations and how to determine if you ought to buy a new set of tires right away.

The basics of tire attrition

The grooves carved in any car tire are called treads. The raison d’être of these treads is to assure that the tires will exert enough traction under any road condition. If your tires did not have such tracks, they would not be able to divert the accumulated rain or snow beneath them, which would inevitably make you hydroplane or skid. Even though treads would probably not be of much help in a world where roads are always 100% dry, they are absolutely essential on Earth, where rain and snow often come disrupt motorists.

When you buy a new set of tires, the depth of these grooves is such that your tires are pretty much suited to drive in any road condition you can legitimately expect to face. In effect, they can easily manage to clear reasonable amounts of rain and snow. However, these grooves become shallower as you use your tires, which also means that their efficacy in specific conditions diminishes, thereby putting you more at risk of being involved in an accident.

Which level of wearing down is acceptable?

If you live in an area where winter is long, cold, and very snowy (i.e., Canada), chances are that you may be forced by the law to have an extra set of tires suited for the winter only. However, if you live in an American state where snowfalls only are occasional events, experts then recommend that you change your tires if the depth of their treads drops under 6/32”.

Rain is probably a more universal kind of precipitation. In effect, every motorist is quite likely to face it from time to time. And since water cannot be compressed, maintaining a certain tread depth is absolutely essential to avoid floating at high speed on accumulated water. Everybody should then know that a tread depth below 4/32” rhymes with high risk on a wet route. In effect, the performance of tires with treads that shallow dangerously drops, putting the driver in an unsafe situation.

Finally, experts also say that when tire treads are only 2/32” deep – which, as noted earlier, is the legal limit in most U.S. states – traction on a road covered with snow is almost inexistent and resistance to hydroplaning is seriously compromised. A motorist should actually avoid wearing down his or her tires that much, because the risk of being involved in a crash greatly increases.

How can I measure tread depth?

In the light of all this, it appears essential to check tread depth once in a while. Such an examination can be performed quite easily by anyone who possesses… American money! In effect, you can put a penny upside down in your tires’ treads, and if you can neither see all of Lincoln’s hair, nor the copper above it, you are somewhat safe: tread depth is at least 2/32”. On the contrary, if you can see all of Lincoln’s head or, even worse, the copper above it, go get a new set of tires right away!

Now, using a penny’s tails sidestill upside down – allows you to make another measurement. If you put it down a tread and some of the Lincoln Memorial’s top is hidden by the tread’s rim, it means that this particular groove’s depth is over 6/32”. Finally, putting an American quarter – still upside down – on the tails side down a tread and having some of Washington’s head hidden means it is over 4/32” deep.

Even though this technique is not infallible and has to be repeated in many different spots on a given tire, it does give motorists an idea of how worn out their tires are. A recent study by the National Highway Traffic and Safety Administration showed that in tire-related accidents, the crash rate of cars with near full tread depth was 2.4%, compared to that of worn out tires (0/32”-2/32”) which was 26%! These statistics certainly are a clear indication of how seriously tire attrition should be taken.

About the author:
Alexandre Duval is a blogger for Hamel BMW dealer. He is also currently completing his master’s degree in political science at the University of Quebec in Montreal.