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When Are Your Tires Worn Out?

Published Jul 28th 2010, 5:24pm by



Hey Los Angeles, are your tires worn out? What is the standard for our CA streets? How can you tell on your car?

While there may be legal requirements for the Los Angeles area, there are safety concerns that go beyond meeting minimum replacement mandates.

2/32 is the depth of the tire tread wear indicator bars that US law has required to be molded across all tires since August 1, 1968. When tires are worn so that this bar is visible, there's just 2/32 of an inch – 1.6 millimeters – of tread left. It's that level of wear that's been called into question recently.

We're referring to the Consumer Reports call to consider replacing tires when tread reaches 4/32 of an inch, or 3.2 millimeters. And the recommendation is backed by some very compelling studies.

The issue is braking on wet surfaces in and around Los Angeles. Most of us think of our brakes doing most of the work, but if you don't have enough tread on your tires, the brakes can't do their job. When it's wet or snowy, the tread of the tire is even more critical to stopping power.

Picture this: you're driving over a water covered stretch of road near Los Angeles, CA. Your tires must be in contact with the road in order to stop. That means that the tire has to move the water away from the tire so that the tire is actually contacting the road and not floating on a thin film of water.

Floating on the surface of water is called hydroplaning. So if there's not enough tread depth on a tire, it can't move the water out of the way and you start to hydroplane.

In the study a section of a test track was flooded with a thin layer of water. If you laid a dime on the track, the water would be deep enough to surround the coin, but not enough to cover it.

A car and a full-sized pick-up were brought up to 70 miles per hour, or 112 kilometers an hour and then made a hard stop in the wet test area. Stopping distance and time were measured for three different tire depths:

  • New tire tread depth
  • 4/32 of an inch
  • 2/32 of an inch

So what happened with the 2/32 tires on the car? Get this – when the car had traveled the distance required to stop with new tires, it was still going 55 miles an hour. Stopping distance was nearly doubled to 379 feet and it took 5.9 seconds.

Wow! That means if you barely have room to stop with new tires, you would hit the car in front of you at 55 miles an hour with the worn tires.

Now, with the partially worn tires – at 4/32 of an inch – the car was still going at 45 miles an hour at the point where new tires brought the car to a halt. It took nearly 100 feet more room to stop and 1.2 seconds longer. That's a big improvement. We can see why Consumer Reports and others are calling for a new standard.

Of course, stopping distances were greater for the heavier pick-up truck.

How do you know when your tires are at 4/32 of an inch? Easy; just insert a quarter into the tread. Put it in upside down. If the tread doesn't cover George Washington's hairline, it's time to replace your tires. With a Canadian quarter, the tread should cover the numbers in the year stamp.

You may remember doing that with pennies. A penny gives you 2/32 to Abraham Lincoln's head. The quarter is the new recommendation – 4/32.

How do people feel about replacing their tires earlier? Well, tires are a big ticket item and most people want to get the most wear out of them that they can. But do you want that much more risk just to run your tires until they are legally worn out?

For us, and we would guess for many, the answer is "no".

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Thousand Oaks, CA 91359
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