An Overview of Tread Depth on Tires

So, when do your tires actually need to be replaced? This is a common question that we may ask ourselves. Many people believe the answer is “when they no longer pass a safety inspection.” However, waiting that long can jeopardize your safety.

The federal government of the United States does not have any laws governing tread depth, but 42 states and all of Canada do. The minimum legal tread depth is two-thirty-seconds of an inch, according to them. Two other states, including California, consider one-thirty-second to be the bare minimum, while six have no standards at all.

Since 1968, all tires in the United States have been required to have a raised bar molded across them. When tires are worn to the point where this bar is visible, there is only 2/32″ of tread remaining. But, does that older standard provide you with adequate protection?

Consumer Reports, on the other hand, has issued a warning to “consider replacing tires when tread reaches 4/32”. And the recommendation is supported by a number of compelling studies. Before we get into the studies, you should be aware that braking on wet surfaces is a major issue.

We tend to think of the brakes as doing all of the stoppings, but effective tires are also required to stop the car. When it’s wet or snowy, tire tread is critical for stopping power.

Consider the following scenario: you’re driving down a flooded road. In order to stop, your tires must make contact with the road. This means that the tire must direct the water away from the road so that the tire does not float on a thin film of water – a condition known as hydroplaning. When a tire lacks sufficient tread depth, it is unable to move water out of the way and begins to hydroplane.

Here’s where the research comes in handy. We believe you will be astounded. A thin layer of water was applied to a section of a test track. If you laid a dime flat on the track, the water would be deep enough to encircle but not cover it.


A car and a full-sized pickup truck were accelerated to 70 mph before coming to a complete stop in the wet test area. Stopping distance and time were measured for three different tire depths. They began by putting new tires through their paces. The tires are then worn down to the legal limit. Finally, tires with a tread depth of 4/32″ were tested – the depth recommended by Consumer Reports.

The car with the legally worn tires was still traveling at 55 mph after braking for the distance required to stop the car with new tires. The stopping distance was more than doubled. That is, if you have barely enough room to stop with new tires, you will collide with the car in front of you at 55 mph with worn tires.

The car was still going 45 mph with the partially worn tires – at the depth recommended by Consumer Reports – when new tires brought it to a halt. That’s a significant improvement, and it’s easy to see why Consumer Reports and others are urging a new standard.

Without getting into too many details, let us tell you that stopping the truck with worn tires required nearly a tenth of a mile of clear road ahead of it to come to a safe stop. Obviously, this is a major safety concern.

The tests were carried out on the same vehicles, but with different sets of tires. Because the brakes were the same, the only difference was in the tires.

So, how do you tell if your tires are at 4/32″? It’s not that difficult. Simply place a quarter in the tread. Insert it backwards. It’s time to replace your tires if the tread does not reach George Washington’s hairline. The tread on a Canadian quarter should cover the year stamp numbers.

You might recall doing something similar with pennies. A penny, on the other hand, gives you 2/32″ of an inch to Abraham Lincoln’s head. The quarter – 4/32″ – is the new standard.

Tires are an expensive purchase, and most people want to get as much use out of them as possible. But are you willing to put yourself in that much more danger just to run your tires until they are legally worn out? The answer is “no” for us, and we’d wager for many others.