How Much Tire Tread Is Left? Reading the Wear Bars

You already own a tread gauge — six of them per tire

Look into the grooves of any tire on a modern car and you will find raised bars running across the tread, set slightly below the surrounding rubber. Those are treadwear indicators, and they are not a manufacturer courtesy. FMVSS 139, the federal standard for light-vehicle radial tires, requires them and specifies both how many there must be and exactly what depth they mark. Once the tread has worn down level with those bars, the tire has reached the depth the standard is built around.

What the standard actually requires

Tread depth advice is mostly folklore built on top of one real number. The real number, and two details that change how you check, are in section S5.4 of FMVSS 139.

• Commonly claimedCheck the wear bars at one spot on the tire and you know where you stand.

  VerifiedThe standard deliberately puts several around the tire so you cannot. S5.4 requires "not less than six treadwear indicators spaced approximately equally around the circumference of the tire" (three, for rims of 12 inches or smaller). That spacing exists because tires do not wear evenly — a suspension or alignment fault, or long-term underinflation, wears one region faster. Check several points around each tire and across its width; if they disagree, the disagreement is the finding.

• Commonly claimedThe bars appear at 2/32 of an inch.

  VerifiedTrue, and the regulation words it differently in a way worth knowing. S5.4 requires the indicators to let an inspector "determine visually whether the tire has worn to a tread depth of one sixteenth of an inch." One sixteenth is the same measurement as 2/32 — the tyre trade counts in thirty-seconds because gauges are marked that way. Same depth, and quoting either is correct.

• Commonly claimedTwo thirty-seconds is the legal minimum tread depth.

  VerifiedNot from this standard, which is about the tire rather than about you. FMVSS 139 governs what the manufacturer must mould into a new tire; nothing in S5.4 sets a minimum depth for using a tire on the road. Minimum legal tread depth is set by individual states and they do not all agree, with some setting a higher figure for steering axles or for commercial vehicles. Treat the wear bars as the point the tire was designed to be judged at, and check your own state law for the legal line.

The penny test, and when to stop trusting it

The coin trick works because it approximates the same depth: with Lincoln head-down in the groove, tread that leaves the top of his head visible is at roughly two thirty-seconds. It is a reasonable field check when you have no gauge. But your tires already carry six purpose-built gauges that need no coin and no interpretation, and a two-dollar depth gauge removes the guesswork entirely. Use the coin to decide whether to look harder, not to decide whether to buy tires.

Wear that is uneven, not just low

Depth is only half of what the grooves tell you. Wear concentrated on both outer edges points at chronic underinflation; wear down the centre strip points at overinflation; wear on one edge only points at camber or a bent component; patchy or scalloped wear points at worn dampers or an out-of-balance assembly. A tire that is legally fine but wearing on one shoulder is telling you to fix something before you spend money on rubber that will wear the same way.

Related references on this site

The date code on the sidewall tells you the tire's age, which matters independently of how much tread is left — a nearly new spare can be a decade old. The tire size decoder covers the rest of the sidewall markings, and the tire pressure reference lists recommended cold pressures, since underinflation is the most common cause of the edge wear described above.

Frequently Asked Questions

Related Guides

• How Old Are My Tires? Reading the DOT Date Code Find the DOT code on your sidewall and read the four digits that give the exact week and year your tires were built — with the federal rule that defines them, quoted.
• Steering Wheel Shakes at Highway Speeds When your steering wheel shakes or vibrates at 55-75 mph but feels fine at low speeds, the cause is almost always a tire, wheel, or suspension issue. Here's how to diagnose it.
• Steering Wheel Pulls Left or Right: 7 Causes Your car pulls to one side when you drive or brake? Learn the seven most common causes, from tire pressure to alignment to brake calipers, and how much each fix costs.