Like the antediluvian Titan upon whose shoulders rest the world, automobile tires support a tremendous load – up to 2,000 pounds per tire. They labor ceaselessly for 50,000 miles, eaten away by winter salt and summer sun, the terror of squirrels, but eventually all rubber is rubbed raw. All tires must be replaced.
These seven signs say when.

Worn Out Tread

The life of a tire is measured not in years, but in sixteenths. Every tire has a tread depth, the distance from the crest of the tread to its root, measured in increments of 1/16 or 1/32 of an inch. In most American states, 1/16-inch is the legal limit.

Tire tread depth can be measured with a tread gauge, wear bars, or a penny. Any ol’ one will do. Insert Lincoln headfirst into a void in the tire. If part of his head is covered, the tire has at least 1/16-inch of tread. Insert a nickel the same. If part of Washington’s crown is covered, the tire has at least 2/16-inch of tread.

Expiration Date

Even an unused tire will succumb to ozone degradation and loss of elasticity. Most auto manufacturers recommend fully replacing or professionally inspecting any tire, regardless of use, after 5-6 years. High-mileage tires used on light trucks and transportation vehicles may be imbued with an anti-ozinant to extend life to 9-10 years.

How to tell the age? Each tire has a serial number embossed onto the sidewall, an identification number, which shows its date of manufacture. The first two digits specify the week of the year; the second two digits show the year.

Failing to replace an aged tire can be lethal. Just ask Roger Rodas, the driver of the Porsche Carrera GT whose crash killed Paul Walker. The tires were nine years old.

Alligator Cracks

When a tire is born, it is enormously flexible, thanks to oil pockets in its molecular pores. As time goes by the oil evaporates. Cracks begin in the sidewall and radiate towards the tread, creating an alligator-like pattern in a phenomenon called “dry rot.” Excessive heat and low inflation accelerate the process.

Fat Bulges

Like humans, tires may fatten as they age. Bulges may develop in the sidewalls, tell-tale signs of the separation of the ply and rubber layers. Most bulges are caused by bouncing into potholes and onto curbs, and can lead to catastrophic failure.

Excessive Vibration

Bumps, thumbs and lumps can signify major problems with a tire, including uneven tread wear and sidewall bulges. Excessive vibration can also rush tire aging by increasing internal temperature and separating the internal layers.

Hasty Deflation

Average Joe passenger tire should lose approximately 1-2 psi per month. Weather extremes hasten the losses. If a tire loses more than 4-5 psi per month, it indicates pinhole punctures, which can result in underinflation, dirt-poor fuel economy and accelerated aging.

Hydroplaning & Snow Spinning

The legal limit for a retired tire may be 1/16-inch, but losses in driving performance arrive much sooner. In one expert 2007 study, a worn-out tire had double the wet stopping distance compared to brand new tires. Chances of hydroplaning skyrocket, and forget about traction in the snow.
Retire and replace tires as they wear out. Bite the bullet before you hit the windshield.