The last time my car was serviced I was told that the tread depth was 4/32" and that the tires should be replaced soon. I had purchased tires at Costco a few years ago (maybe more than a few years? Days and months and years tend to speed by with age). Surely my tires didn’t need replaced already. I visually inspected the tires and they looked fine. But I also had no clue what to look at.
Fast forward a few months… I planned to take a road trip so decided to get new tires. Costco has great prices on tires, but in my experience takes at least two weeks to order and install them. I didn’t have that much time. So I caved and bought a set of tires from a local auto repair chain, who procured and installed new tires in two days. I paid significantly more than I would have at Costco. But I also got new tires when I thought I needed them. Did I really need them? Not sure – so I decided to educate myself about tire tread depth so that I could measure it myself rather than relying on auto repair shops.
Tire Tread on New Tires
What is tire tread?
Tire tread is the “rubber on [the tire’s] circumference that makes contact with the road or the ground. As tires are used, the tread is worn off, limiting its effectiveness in providing traction.” (per Wikipedia, see References below).
The purpose of the tread is to help the car “stick to” the road. When you slam on the brakes to avoid hitting something, or corner faster than you should, the tire’s tread helps the car stick to the road and not skid.
The grooves in the tread are designed to remove water from underneath the tire and thus prevent hydroplaning. Hydroplaning is when water gets underneath tires and causes the tire to skid. Too little tread and a car will slip and slide when it rains – especially if it is driven erratically (i.e. quick stops, starts, turns). Of course, Retired Engineer drives like the old man he is, so never drives erratically.
How much tire tread do I need?
In the US, tire tread is measured in 32nd of an inch. The less tread your tire has, the longer the stopping distance on a wet road. Stopping distance is how far your car travels from when you slam on the brakes until the car stops.
Per Tiregrades.com (see References below), a tire tread depth of 5/32" or more is considered safe. Tires with a tread depth of 4/32" or less should be replaced (which aligns with the warning I received when my car was serviced). Tires with 2/32" should be replaced immediately, are unsafe, and will fail an auto inspection.
How can I measure my tire’s tread depth?
Methods including sticking a penny or quarter into the tread with the president’s head pointing towards the tire. For a penny, the distance from the edge of the coin to the top of Lincoln’s head is 2/32". If you can see the stop of Lincoln’s head, you need to replace your tires ASAP. Using the same technique with a quarter, the distance is 4/32" from the edge of the coin to Washington’s head. So if you see the top of Washington’s head, your tires need replaced soon.
A better approach is to buy a tire depth gauge. The photo below shows the tire tread gauge I recently bought. I like this gauge because it is easy to read (no squinting at penny’s or fine print). It is very easy to use. Simply push the rod into the tire groove until it contacts the tire, then read the depth from the dial.
Measuring with Jaco TreadPro Tire Tread Depth Gauge, Dial Type
Every tire I’ve ever seen has grooves in the tire of varying depths – some shallow and some deep. Which type of tread should I measure? The answer is that you should measure the deepest groove on the tire (aka the “main” groove), not the smaller, shallower grooves.
Note that most tires have “wear bars” in the main groove(s) of the tires. These are at 2/32" so if your tire tread wears down to the depth of the wear bars, you should replace them immediately. The photo above clearly shows the wear bars in the main groove of the tires being measured.
Measured depth of 9/32"
Uneven wear and tire rotations?
Have you ever been asked whether you want your tires rotated? Have you ever done it? I almost always say “no”. But perhaps I should rethink this answer.
Misaligned tires cause:
- Uneven tread wear
- Car pulling to one side (you need to constantly steer slightly left or right to keep the car going straight)
- Excessive vibration of steering wheel
To check for uneven tread wear, use your tire depth gauge to measure the depth in each major groove on each tire. All measurements should be identical or nearly so. If you notice specific grooves in specific tires showing additional wear, having your tires rotated will prolong the life of the tires. Alternately, you could get your tire alignment checked.
Conclusion
This retired engineer is by no means an expert on cars or tires. But it is never too late to learn! If you want to learn even more about tire tread, stopping distances, tire alignment , etc – see the articles below.
References: