Your tires’ tread plays a crucial role in performance and safety when you drive. It grips the road surface so your car can move forward and stay on the road in slippery conditions. It also channels water out from under your tires to keep you from hydroplaning in rainy conditions. But with use, tread wears down, compromising all of these essential functions. This wear can be gauged by measuring tread depth, which is the vertical distance between the highest point on the tread’s pattern to the lowest point in the pattern’s grooves. Tread depth will tell you whether or not you need to replace your tires to avoid dangerous loss of traction.
The most common of several methods for taking this measurement are: checking tread wear indicator bars, using the penny test, or measuring with a tire tread depth gauge. Whichever method you use, you need to check the tread depth at various points around the tire’s circumference. You should use the smallest measurement as the basis for your decisions about replacing tires.
Maybe the easiest way to check tread depth is to examine the tread wear indicator bars. These bars are chunks of rubber that span the grooves in the tread at different points around the circumference of the tire.
Their height is 2/32nds of an inch above the lowest point of the groove, which is the depth at which the U.S. Department of Transportation recommends replacing a tire. If your tire has worn down until flush with any of the tread wear indicator bars, it’s time to replace it, even if the tread is still higher than the indicator bars elsewhere on the tire.
To use this method, take a penny and hold it so the engraving of Abraham Lincoln’s head points downward. Then insert the penny into one of the tread’s grooves, with the head side facing you. Crouch down so your eyes are level with the surface of the part of the tread you are measuring.
If you can see a space between the top of Lincoln’s head and the tread, or if the tread is flush with the top of Lincoln’s head, this means your tires are worn to the point that they are no longer safe and need to be changed. Check different parts around tire, and if any point fails the test, get new tires.
Another way to check your tread depth is to use a tire tread depth gauge. The most common model of this tool is a graduated probe, which you can find at auto parts stores. The graduated probe has a thin, retractable rod, a perpendicular “foot,” and a cylinder with measurements marked on it.
To use the gauge, fully extend the rod and insert it into one of the grooves. Then push down the “foot” of the gauge until it firmly rests on two elevated ribs of the tread on either side of the groove. The shaft with the measurement markings will slide upward as you push the “foot” down and show you the depth of the tread. This shaft usually displays the measurements in both 32nds of an inch and millimeters. Note the depth and repeat the process at various points on the tire, taking the smallest reading as the definitive one.
While there is no national law in the U.S. specifying minimum legal tread depth, most state laws require a tire to have at a tread depth of at least 2/32nds of an inch. Likewise, the U.S. Federal Motor Vehicle Safety Standards require all tires to be manufactured with a visual indicator allowing drivers to know when the tread has worn down to 2/32nds of an inch or less. However, the actual amount of tread needed to safely drive and stop a vehicle in different conditions may exceed this technical minimum.
One of the main functions of the grooves in tires is to push water or slush out from under it, or to bite into snow-packed roads, and shallow grooves do not do this job nearly as well as deeper ones. Studies also show that traction and stopping power drop significantly from a tire with 4/32nds of an inch of tread to one with 2/32nds of an inch, and most experts agree that it is best to begin looking to replace tires when they fall below the larger of the two depths. This is especially true if you habitually drive in wet, slushy, or muddy conditions. In snowy conditions, adequate tread depth is even more important.
A few key numbers to keep in mind when checking tread depth are:
Tread depth and stopping distance have an inverse relationship: the shallower the tread, the longer it will take for a vehicle to stop. Exact differences in stopping distance will vary based on a range of factors (type of tire, type of vehicle, road conditions, etc.). However, in many cases a tire with a tread depth of 2/32nds of an inch will take nearly twice as long to stop on a wet roadway as a new tire with full tread depth. A tire with a tread depth of 4/32nds of an inch will generally stop half again as fast as a new tire. This means stopping power drops off exponentially below 4/32nds of an inch, which is why many advocate for replacing tires before they reach the legal limit set by most states.
While tire tread will inevitably wear down over time, there are several measures you can take to slow this process and keep the tread deep and effective:
Heel and toe wear, or heel-toe wear, is when the leading edge of each tread block wears down more quickly than the trailing edge. As a result, the blocks form a series of miniature ramps around the circumference of the tire. It gives the tire a silhouette similar to that of a circular saw blade, with its teeth smoothed over on one side and sharp on the other. This type pattern indicates accelerated wear and can cause tire damage. It can usually be prevented or remedied by rotating tires.
The penny test is an accurate way to measure whether tire tread depth surpasses or falls short of 2/32nds of an inch, since 2/32nds of an inch is the universally standard distance between the top of Lincoln’s head and the edge of the coin (see above for an explanation of how to do the test). However, many tire professionals now recommend replacing tires when their tread depth reaches 4/32nds of an inch instead of 2/32nds. They suggest using the quarter test instead, since the standard distance from the top of Washington’s head to the edge of a quarter is 4/32nds of an inch.
If your tire’s tread is too shallow to be safe or you have other questions about your tires or tread, contact your nearest tire dealer.
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