Everyone knows the penny test for tread depth. But tread depth is only one of five things that determines whether a tire is at the end of its life. Here are the others, in roughly the order that they tend to retire a tire in the real world.
1. Age — six years from the DOT code
Rubber dries out and hardens over time, regardless of how many miles are on the tire. Most manufacturers and the NHTSA recommend replacing tires that are six years old from their DOT date code, and ten years is treated as a hard ceiling. A tire with plenty of tread but a 2017 build date is at the end of its life today.
2. Visible cracks in the sidewall or tread grooves
Look at the sidewall in good light. Run your fingernail across it. Small, fine 'crow's-foot' cracks are normal weathering on an older tire and not immediately dangerous. Deep cracks that you can fit your fingernail into are dry rot — the rubber compound has lost its plasticizers and is structurally compromised. Replace immediately.
3. Vibration that wasn't there a month ago
A vibration that starts at a specific speed (say, 60 mph) and gets worse at higher speeds is usually a balance issue and fixable. A vibration that's present at all speeds and felt through the seat as much as the steering wheel is often a belt that has shifted inside the tire. That tire is unsafe at highway speed.
4. The car pulls or steers unevenly
If the car pulls to one side and a four-wheel alignment doesn't fix it, you likely have a 'radial pull' — a manufacturing variation or damage in one tire. Swapping that tire side-to-side will move the pull in the opposite direction and confirm. Replace the bad tire.
5. Wear bars are showing
Inside the tread grooves, look for small raised bars running across the grooves. These are the manufacturer's wear bars and sit at 2/32 of tread depth. If they're flush with the surrounding tread, the tire is at the legal wear limit. Most tire shops won't let you leave with a tire that's reached its wear bars.
The 4/32 rule for wet roads
Legally a tire is worn at 2/32. But Consumer Reports and most safety engineers recommend replacement at 4/32 if you drive on wet roads, because hydroplaning resistance drops off a cliff between 4/32 and 2/32. If you live anywhere it rains, treat 4/32 as your number.
One sign on its own isn't a death sentence. Two or more is the tire telling you it's done.