Rick's Free Auto Repair Advice

When to replace tires

when to buy new tires

Here’s when to replace tire

Replace your tires when they wear down to 4/32″

The legal limit for tire tread depth is 2/32″, but you shouldn’t wait until they’re that worn. Here’s why.

The safety studies show a dramatic decrease in stopping power once you wear down to 4/32″ and below

tire wear barsIndependent tests done by Tirerack.com and other consumer testing organizations prove that once tires wear to 4/32” it takes an additional 125-feet to stop on wet pavement than a new tire.

That statistic alone should convince you that you need to keep an eye on tread depth.

Worse yet, once your tires wear down to the legal limit of 2/32” your stopping distance increases by up to 250 additional feet on wet pavement than a new tire. Watch this video from Tirerack.com to see the test results.

Forget about the penny test, get a real tread depth gauge

If you do a search on the Internet, every link you get will tell you to insert a penny into several grooves on your tire with Lincoln’s head in the deepest part. If Lincoln’s head is covered by tread, it means you have more than 2/32” tread left.

That technique is one of the dumbest ideas I’ve ever heard of. Here’s why. tire wear penny testFirst, all tires have built in wear bars (see photo below). When the wear bars are even with the tread, you’re at the legal limit of 2/32”. So you really don’t need a penny to discover that. Second, if you do the penny test and Lincoln’s head isn’t covered by tread, you’re already driving on unsafe tires. Didn’t you want to know that a little earlier in the game? And, if the rubber does cover Lincoln’s head does that automatically mean you don’t need new tires? NO!

So the time to buy new tires is when your tires reach 4/32″, not 2/32″.

The best way to determine the remaining tread depth on your tires is to buy an inexpensive tread depth gauge. Find one at any auto parts store for less than $10. Keep it in your glove box. Then use it a few times a year to track how well your tires are wearing and to stop inflation and alignment problems.

tire tread depth gaugeHow to use a tread depth gauge

Wipe dirt off the top of the tire. Extend the tread depth gauge probe and insert it into the groove in the middle of the tire. Read the depth and write it down on a notepad. Repeat the test on the outer most and inner most groove. If the tire is wearing evenly and the vehicle is in alignment, all three readings should be within 1/32” of each other. If they’re not, refer to the images below to find out what’s going on. Repeat this test on all four tires.

What other factors go into making a decision to buy new tires?

The tire’s age is another important factor. Rubber deteriorates from the sun’s tire age cracksUV rays and from excessive heat buildup due to under inflation. As the rubber deteriorates, it forms small cracks. Click on the tire to the left so you can see the cracks.

Those cracks can result in unexpected failure at high speeds. So it’s important to inspect your tires for proper air pressure and for cracks. If you see any, it’s time for new tires regardless of the remaining tread depth.

Next, check the tire’s age using the DOT’s “born on” date. Find the DOT stamp on the tire and scan to the last four digits in the code. The last two digits indicate the year it was manufactured, and the proceeding two digits indicate the week it was manufactured.

tire date codeThis DOT indicates that the tire was built in the 50th week of 2008 DOTB9YRUJNX5008

Tires have a maximum useful life of just ten years, regardless of the remaining tread—even if the tire shows no signs of cracking. After five years, tires should be inspected at least once a year for signs of deterioration.

How to tell what’s wrong by checking the tire wear pattern

tire wear chart

Other tire wear photos that indicate problems

Cupping caused by worn shock absorbers or struts

tire cuppingThe shock/strut’s job is to dampen spring rebound. When they wear, the tire bounces off the ground and slows while in the air. When it hits the ground, the pavement scrubs off tread. Once that starts the tire becomes unbalanced and the tire will hit in the same spot on every bump. Eventually the pattern becomes visible and the tire is ruined.

Heat cracks caused by running the tire when it’s low on air
When you drive on a tire that’s under inflated, you create excessive flex at the tire’s shoulders. The heat buildup causes the rubber to deteriorate and crack, eventually causing it to fail. Just hope that doesn’t happen in a turn.

©, 2016 Rick Muscoplat





Posted on by Rick Muscoplat


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