Tire Treads

The Weekly Safety Meeting for July 26th is Motor Vehicles (Vol. 33, Issue 30). In the second column of that meeting, under the heading “Maintain” we recommend using a quarter to measure the depth of tire treads. You may be more familiar with using a penny for that measurement. The reasons behind both tests are briefly described on this page. Below are links to two versions of the Motor Vehicles meeting, one with the penny test and one with the quarter test, so you can use the version you are most comfortable with.

Quarter Test Penny Test

For many years the simple and common way to determine if your tires needed to be replaced was to use the Lincoln Penny Test. To check your tire treads with a penny, you place a penny in the tread groove with Lincoln’s head pointed toward the axle. If you can see the top of Abe’s head, you need to replace that tire.

In many, if not all, states your tires are considered worn out if the tread depth is 2/32 of an inch or less. It just so happens that the distance from the edge of a penny to the top of Abe’s head is very close to 2/32 of an inch. So, if you can see the top of his head, you have less than 2/32 of an inch of tread depth. The Lincoln Penny Test is a simple and quick way to see if your tires are legal.

There is very often a difference between legal and safe. The Washington Quarter Test is now considered to be a much safer measure of tread wear. The concept is identical, you place a quarter in the tread groove with George’s head toward the axle. If you can see the top of his head, you have less than 4/32 of an inch of tread remaining on your tires.

Both Consumer Reports and The Tire Rack (a leading, private distributor of high performance automobile components) have performed independent tests on tires. They both concluded that stopping distance on wet surfaces increases dramatically when tires are worn down to only 2/32 of an inch (the legal limit). And they recommend that anyone who drives in rainy conditions seriously consider replacing tires that have less than 4/32 of an inch of tread remaining.

You can see the results of their testing at the links below:

   The Tire Rack:   http://www.tirerack.com/tires/tests/testDisplay.jsp?&ttid=85
   Consumer Reports:   http://www.consumerreports.org/cro/cars/tires-auto-parts/tires/tire-buying-advice/tire-tire-care/tire-tire-care.htm

Tires with less tread depth allow less water to squeeze out through the tread grooves. With nowhere to go, that water sits between the rubber and the road and prevents the tire from gripping the pavement. The result is poor handling and longer stopping distances on wet surfaces.

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