by Frank Lobsterman

Created on: July 23, 2008

Tires are arguably the most important part of a vehicle. When they are worn out or improperly inflated, they may have a negative impact on every aspect of a vehicle's performance. Because of this, keeping an eye on tire wear is of utmost importance to anyone who values safety at all. Proper inflation is critically important to tire performance but that is not the subject of this article. I will only touch on inflation briefly as it relates to tread wear. This is about tread depth and how to know when it's time for new tires.

The legal minimum tread depth in most areas is 2/32". Mechanics will use a more precise measuring device, but for the average person to know when a new set is in order, the following penny trick will suffice. Stick a penny between tread blocks, with Abe Lincoln head down in the tire. If you can see the top of his head, it's time for tires. Be sure to check the tread depth at the inner and outer edges, as well as the center of the tire. Uneven tread wear may indicate that the vehicle is in need of an alignment or the tires are improperly inflated. I will expound on that later.

I usually just use the aforementioned penny trick to judge whether I need tires or not, but if you really want to keep a close eye on things, there are some more measurements that can be done with coins. The distance on a quarter from the top of the coin to the top of Washington's head denotes 4/32", and the distance from the top of a penny to the top of the Lincoln memorial is 6/32". You may want to use these tricks to estimate when you will need new tires long before they become necessary, but changes in tire depth in this range don't usually impact performance under normal driving conditions.

Back to uneven tread wear. Most cars will develop some uneven wear if the tires are not rotated regularly, but if it's worse than usual, it can be a sign of something more that is likely having a negative effect on gas mileage. If a tire is wearing heavily on the outside or inside edge but not both, it's probably an alignment issue. If the tire is wearing heavily on the center tread, it's probably overinflated. Conversely, if it's wearing heavily on both the outside and inside treads, it's probably under-inflated. All of these conditions will eat through tires faster than normal and make the vehicle unsafe to drive. Under-inflation and poor alignment will also decrease gas mileage. This is all kind of common sense for a physics-minded person, but it may be a lot to learn for everyone else. Just remember that uneven tire wear is the result of one of two things. Check your tires for proper inflation, and if that's not the problem, it's probably time for an alignment.

Now that you know what to look for and how to look for it, be sure to do it regularly. The earlier you can detect a problem, the less it will cost you in the long run. A well-maintained set of tires will save you money and help to keep you out of trouble on the road.

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