Reading a tire

There are two readings to take. The sidewall tells you about the tire — what it is, what it's rated for, how old it is. The tread tells you about the car: the pressures it's been run at, the alignment it's carrying, the state of the suspension holding it up. The second reading is the valuable one, and almost nobody takes it.

The sidewall

Everything molded into a sidewall is there because a standard put it there, which is why it reads like a parts catalog exploded. Four things on it matter.

The size code

Read 225/65R17 left to right and it stops being cryptic. Each segment answers a different question about the shape of the tire.

The group after it — 102H here — is a load index and a speed rating. The load index is a code, not a weight in pounds; it maps to a figure on a standard load table, so look it up rather than guess. The letter is the maximum sustained speed the tire was certified to under test conditions — which matters less for how you drive than for making sure a replacement matches what the car was built around.

That reference point is the one people miss. The car has an opinion, printed on a placard in the driver's door jamb: the size, load rating and cold inflation pressures it was engineered for. The placard is authoritative; what's bolted on now is what the last owner bought. And the pressure molded into the sidewall is the tire's maximum, not the car's — inflating to it produces the first wear pattern below.

The DOT code, and why age is its own problem

At the end of the DOT string sit four digits: the first two are the week, the last two the year. 3624 is the thirty-sixth week of 2024. If you find no four-digit group, check the other side of the tire — the complete code is only required on one sidewall, and cars are often mounted with that side facing in.

Age matters independently of tread depth, and that surprises people. Rubber keeps reacting after it leaves the factory: oxygen, ozone, ultraviolet light and heat cycles all work on it whether the car is driven or parked, and the compound hardens from the inside out. A tire can have nearly all its tread and still be past its useful life — which is exactly the low-mileage car that spent six years in a driveway.

How old is too old is not a figure we're going to invent. Manufacturers and safety organizations publish guidance, it differs between them, and it's framed as guidance rather than a rule. Read the date code, then ask a tire professional what they'd do with rubber that age. Check the spare too — usually the oldest on the car.

Tread depth

Depth is measured in thirty-seconds of an inch and you can get close enough without a gauge. Put a penny in a groove with Lincoln upside down: if the top of his head shows, you're at or below 2/32, which is where the molded wear bars sit. Those bars are the raised strips across the bottom of the grooves; when they come level with the tread, the tire has reached that point. A quarter, Washington upside down, marks roughly 4/32. Legal minimums are set by state and change, so check New Jersey's current rule or ask a shop.

The more important habit is where you measure: outer edge, center and inner edge of each tire, at two or three points around it. One reading in the middle gives you a number. Three across the tread tell you what the car has been doing.

What the wear pattern says about the car

This is the payload. A tire wearing evenly reports that pressure, alignment and suspension are all doing their jobs. A tire wearing unevenly is not a defective tire — it's an instrument, and it's pointing at something else on the car.

Figure 2. Four cross-sections through the tread. The dashed line is the original profile; the solid shape is what's left. Exaggerated for legibility — real wear is subtler, and easier to feel than to see.

Center wear means the tire has been run over-pressure, which bows the tread outward so the middle does the work. It's the cheapest problem here — but it also means nobody has been checking pressures, which says something about the rest of the maintenance.

Both shoulders is the opposite: too little air, the tread sagging in the center so the car rides on its edges. Chronic underinflation is more than a wear issue — it builds heat in the sidewall, and heat is how tires fail at speed rather than gracefully in a parking lot. Hard cornering wears the outer shoulder alone, so read the pattern against how the car has been driven.

One shoulder is alignment, and it's the pattern that costs most to ignore, because whatever is misaligned will chew through a brand new tire at the same rate. Slight extra inner-edge wear at the rear is normal by design on some cars. A pronounced taper, or the same taper on both fronts, is a toe or camber problem — or a control arm, strut mount or tie rod that's had enough. A car that came back from an impact and was never properly realigned wears its tires like this.

Cupping is the diagnostic prize. Scalloped high-and-low patches around the circumference mean the wheel isn't being held down consistently: worn shocks or struts letting it bounce, a bad balance, tired bushings. The tire is the symptom, and replacing it fixes nothing — the new one will cup too, on the same schedule.

A fifth pattern you feel rather than see: run a flat palm across the tread from the inside edge outward, and if the blocks feel smooth one way and sharp-edged the other, that's feathering, and feathering is toe.

Standing in front of a car

Four different brands on four corners means somebody replaced tires one at a time as they failed. Four matching tires with an old date code on a low-mileage car means the rubber is older than the miles suggest. Brand new tires on a high-mileage car can be good maintenance — or a wear pattern somebody would rather you didn't read.

Check pressures cold against the door placard, read all four date codes and the spare, and put a palm across each tread. Ninety seconds, and it tells you more about how a car was kept than any paragraph in a listing — including things no history report will show you.

General information, not a substitute for inspection by a qualified tire professional. Legal minimum tread depths are set by state and change; age-related replacement guidance differs between manufacturers and safety organizations and is not uniform. The codes in Figure 1 are illustrative and describe no actual tire or specification.