When to replace your tires

How much tread is enough

Tread depth is measured in 32nds of an inch. New passenger tires usually start around 10/32 or 11/32. At 2/32 a tire is worn out: that is the point NHTSA uses, and the usual legal minimum.

The grooves in the tread are what carry water out from under the tire. As they get shallower, the tire moves less water and grips less on a wet road, well before it reaches 2/32. That is why many tire retailers suggest replacing tires around 4/32 for wet traction. It is their guidance, not a law, but it is worth taking seriously in a city that floods.

Hydroplaning and flooded roads

The penny test

1. Hold a penny with Lincoln's head pointing down and his face toward you.
2. Push it head first into one of the main grooves.
3. If you can see the top of his head, the tread is at or below 2/32. That tire is due.
4. Repeat on the inside, center and outside grooves of every tire, and at a few spots around each one.

Check more than one spot. Tires rarely wear evenly, and the inside edge, the one you cannot see from the curb, is often the first to go.

The penny tells you a tire is done. It does not tell you a tire is fine. If you want an actual number, ask us to measure the tread when you stop in.

Wear bars in the grooves

Most tires have small raised bars molded across the bottom of the main grooves, set at 2/32. On a tire with good tread they sit low and are easy to miss. When the tread wears down flush with them, they show as solid strips running across the tread, and the tire is worn out.

If you can see a bar level with the tread anywhere on the tire, treat the whole tire as done, even if other spots look deeper.

What the wear pattern tells you

Look across the whole width of each tire. Even wear means the tire simply got used. Uneven wear has a cause, and if the cause is not fixed, the next set wears the same way.

If one edge is going first, have the alignment checked before the new tires go on. Otherwise you pay for the same problem twice.

Wheel alignment, with a before and after printout

Cuts, cracks and bulges

Tread is not the only reason to replace a tire. Damage to the sidewall can end a tire with plenty of tread left, and sidewall damage cannot be repaired.

Cracking is the rubber breaking down. Fine surface lines and deep cracks are different problems, so let us look at it rather than guess. A tire that keeps losing air may only have a nail in the tread, which can often be repaired.

Flat repair: when a tire can be fixed

Make it part of a monthly check

NHTSA suggests looking your tires over once a month, the same time you check the pressure. It takes a few minutes in the driveway.

• Run the penny test on each tire, inside, center and outside.
• Look along both sidewalls for cuts, bulges and cracks. Turn the steering wheel all the way to one side to see the inner edge of the front tires.
• Look for nails, screws or anything else stuck in the tread.
• Compare the tires with each other. One that is wearing faster than the rest is telling you something.

If something changed since last month, a nail that was not there or an edge that suddenly looks thin, come in then. It is a lot easier to deal with before the tire goes flat on the freeway.

Replacing two tires or four

Replacing all four keeps grip and handling even from corner to corner, but it is not always necessary. If two tires are worn and two still have plenty of tread, a pair can make sense on many front-wheel and rear-wheel drive cars. The two on the same axle should match each other in size and type.

All-wheel drive is different. These systems can be sensitive to tread depth differences between tires, and some makers set a small limit on how far apart the tires can be. Your owner's manual says what your car needs. Follow it, even when two tires would be cheaper today.

Bring the car in with the size written down, and tell us how much tread the tires staying on the car have left. We will help you match the size and type.