Tread lets a tire grip on wet and snowy roads, and it wears down so gradually that it's easy to overlook. A few quick checks show how much is left, and regular rotation helps the set wear evenly.
How tread depth is measured
In the US, tread depth is measured in 32nds of an inch. According to Bridgestone, new tires typically start at about 10/32 or 11/32 inch, and some truck, SUV and winter tires start deeper.
The legal minimum and a safer margin
NHTSA advises replacing tires once the tread is worn to 2/32 inch, and AAA notes that tread below 2/32 inch falls under the legal minimum in most states. Rules vary by state, so check your state's requirements.
That minimum is a floor, not a goal. In AAA research, passenger cars on all-season tires worn to 4/32 inch needed an average of 87 more feet to stop on wet pavement than on new tires. AAA suggests shopping for new tires once tread gets down to about 4/32 inch.
The penny and quarter tests
- Penny test (2/32 inch): Place a penny in a tread groove with Lincoln's head pointing down. If you can see his entire head, the tread is at or below about 2/32 inch and the tire is due for replacement.
- Quarter test (4/32 inch): Do the same with a quarter, Washington's head down. If the tread covers part of his head, you have at least 4/32 inch. If you can see all of it, the tread is getting shallow for wet roads and it's time to plan for new tires.
Tread wear bars
Tires also have built-in tread wear indicators: small raised bars molded into the bottom of the grooves at 2/32 inch. When the tread has worn level with them, they show up as flat strips running across the tread, and the tire should be replaced.
How often to rotate
Each wheel position wears a tire differently. On a front-wheel-drive vehicle, for example, the front tires handle a larger share of the forces from steering, accelerating and braking. Rotation moves each tire to a new position so the whole set wears more evenly.
Common guidance falls around every 5,000 to 8,000 miles. NHTSA uses that range for vehicles whose makers recommend rotation, Bridgestone gives the same range or every six months, and Michelin cites 6,000 to 8,000 miles as a common recommendation. Your owner's manual sets the interval and pattern for your vehicle, so start there.
Some setups have limits. Directional tires generally stay on the same side of the vehicle, and vehicles with different tire sizes front and rear may not allow a standard rotation, or any rotation at all.
Signs of uneven wear
Look across the full width of each tread. These patterns point to common causes:
| Wear pattern | Common cause |
|---|---|
| Center of the tread worn more than the edges | Overinflation |
| Both outer edges worn more than the center | Underinflation or a slow leak |
| Heavy wear on the inner or outer edge only | Wheel alignment out of specification |
| Feathered or scalloped tread blocks | Alignment settings, worn suspension parts, skipped rotations or internal tire damage |
If the pattern points to pressure, correcting it and checking monthly helps keep the wear from getting worse. The other patterns call for a professional inspection.
When to get an alignment
An alignment adjusts the angles of the steering and suspension, not the tires themselves, and it takes special equipment and a qualified technician. Have it checked if you notice any of these:
- The vehicle pulls left or right on a straight, level road
- The steering wheel is off center when you drive straight
- Uneven tread wear with no other explanation
- Vibration in the steering wheel, which can also mean the tires need balancing
AAA also recommends an alignment check whenever new tires are installed, after a hard hit to a pothole or curb, and after steering repairs or a collision. Worn suspension parts can contribute to misalignment, so the technician may spot related problems during the check.
Monthly tread checklist
- Do the quarter test in several spots on each tire, including the inner and outer grooves.
- Go by the most worn spot, since tires don't always wear evenly.
- Look for wear bars that sit level with the tread.
- Compare any uneven pattern with the table above.
- Check your mileage against the rotation interval in your owner's manual.
To keep pressures on target between rotations, see the tire inflators in our Jump Starters & Tire Inflators collection.
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