Car tires, while impressively durable, will not last forever. As they roll on the road, microscopic amounts of rubber are torn off the tread, reducing its depth. Shallower tread equals weaker grip on wet surfaces and, in case of winter tires, reduced grip on snow. While less tread depth may slightly improve traction on dry surfaces due to a higher contact area, it will dramatically decrease safety in rain.
Typically, a summer, passenger car tire begins with a 10/32” (7.9 mm) tread depth. Tires are considered “bald” when the tread is reduced to 2/32” (1.6 mm), which is the legal minimum in most states. At about 4/32” (3 mm), you should consider replacing your tires, since at this depth, the tread's ability to evacuate water is decreased. It is recommended to check the tire tread depth at least once a month. There are several methods of measuring the tread depth that you can use.
Tread worn to 2/32" depth - this tire is dangerous to use and has to be replaced
Tire tread depth gauges
The best method to check how much tread you have left – is to visit a car service. You can also check it yourself – it is easier to do it professionally than you might think. In car part shops you can purchase a simple device called tire tread depth gauge, which looks a bit like a metal syringe without a needle. Before you use it on a tire, press the probe against a flat surface to make sure it works correctly – fully pressed should show zero depth.
Place the probe into the main central groove of the tire and press the device so that its base touches the blocks in the tread. The gauge will measure the distance between them, giving you a good measurement of your tread depth. Remember to repeat the measurement in other grooves in the tread – one check is not a serious measurement, but an average of more of them should give you an accurate reading.
What can influence the reading is the tire pressure. Incorrectly inflated tires will cause an uneven treadwear. Overinflated tires will wear more in the center, while underinflated tires put more stress on the sides of the tread. Bear that in mind when you conduct your test – with a tire tread depth gauge you can detect a problem with tire pressure much earlier than with other tests.
The coin method
Another popular method of checking how much tread you have left is the coin method. How to check tire tread with a penny? Insert the coin into several the central groove. If Lincoln’s head does not touch the tread even in a slightest degree – you have more than the minimal 2/32” of tread depth left to legally use your tires. The same method used with a quarter will give you an estimate on 4/32”, below which you should consider changing your tires.
This method is good to try on the fly, in lack of a better one, but will do the job. Its results may be influenced by the position from which you look at the coin, so it is recommended to slightly press it against tread block, but be careful not to put the coin at an angle.
In grooves of the tread, you may spot shallow bars between some blocks. Most tires will have six or more of them. These are inbuilt tire tread wear indicators – remember where they are. If the tire tread is worn enough to reduce the tread to the height of these bars, you reached the 2/32” limit. Such tires are considered bald by the law and need to be replaced.
Other types of tires
Remember that the methods of checking the tread depth described above apply to summer, passenger car tires. Other types of tires will have different standards that apply to them.
Racing tires, called slicks, have close to no visible tread pattern, which ensures fantastic grip on dry surfaces due to maximum contact area with the ground. However, they will fail even on a slightly wet road, and begin to slide. Most slicks are single-use – extreme conditions during a race can tear the tread in a single race.
Winter tires, on the other hand, are made to improve grip on wet surfaces and those covered in snow, at the cost of higher rolling resistance, what makes them less fuel-efficient. Such tires generally have deeper treads and need them to stay that way to guarantee safety on mud and snow, so you should plan to replace your winter tires if you have less than 6/32” of tread depth remaining.
Other types of tires may also have different requirements. For example, the minimum tread depth for a steer tire of a commercial truck is 4/32” – on every tread groove. One groove below this limit will render the whole tire illegal to use. Minimal tread depth for drive tires is the regular 2/32”, and the same applies to trailer tires.