Tire tread depth is the vertical measurement, in 32nds of an inch, between the top of a tire’s tread and the bottom of its grooves. It’s the single number that tells a driver or a fleet manager how much grip, water evacuation, and stopping power a tire has left. In the United States, tread depth is also a compliance number: federal rules set a hard floor for commercial tires, and falling below it during a roadside inspection can pull a truck out of service on the spot.
KEY TAKEAWAYS
- Under 49 CFR 393.75, steer tires on commercial trucks and buses need at least 4/32 inch of tread in every major groove, while drive and trailer tires need at least 2/32 inch.
- Passenger vehicle tires are legal at 2/32 inch nationwide, though a few states set their own inspection thresholds.
- A 2018 AAA study found that tires worn to 4/32 inch need an average of 87 additional feet to stop on wet pavement compared to new tires, a 43% increase in stopping distance.
- CVSA’s roadside out-of-service threshold is stricter than the federal minimum: a steer tire under 2/32 inch or any other tire under 1/32 inch puts a vehicle out of service immediately.
- A tread depth gauge, a wear bar check, or a coin test all work for a single reading, but only continuous monitoring catches uneven wear and slow leaks between manual checks.
In this guide, we cover the tread depth thresholds that matter, when to replace a tire before it becomes a violation, how to measure tread depth correctly, why the number affects more than compliance, and how fleets are moving past manual coin checks toward continuous tire monitoring.
Quick answer
| Question | Answer |
| Legal tread depth | 2/32 inch for passenger tires and most commercial positions, 4/32 inch for steer tires |
| Recommended replacement | 4/32 inch for reliable wet-weather grip |
| New tire depth | 10/32 to 20/32 inch, depending on tire type and application |
| Measurement unit | 32nds of an inch in the US, millimeters elsewhere |
Tire tread depth chart
| Tread depth | Condition | What it means |
| 0/32″ to 12/32″ | New tire | Full traction and water evacuation in all conditions |
| 8/32″ | Good | Strong wet and dry performance, suited to off-road or heavy-load use |
| 6/32″ | Fair, monitor closely | Minimum safe depth for snow and ice; traction starts declining below this point |
| 5/32″ to 4/32″ | Replace soon | Wet-weather grip drops sharply; the recommended replacement threshold for most drivers |
| 3/32″ | Marginal | Legal for most positions, but well below the safety buffer fleets should carry |
| 2/32″ | Legal minimum (most positions) | Wear bars are flush; the tire is at the federal floor and should be replaced immediately |
| Below 2/32″ | Illegal, out of service | Fails DOT and CVSA criteria; vehicle cannot be operated until replaced |
Tread depth in millimeters
| Inches | Millimeters |
| 2/32″ | 1.6 mm |
| 4/32″ | 3.2 mm |
| 6/32″ | 4.8 mm |
| 8/32″ | 6.4 mm |
DOT and FMCSA tire tread requirements
The federal standard for commercial motor vehicles comes from 49 CFR 393.75, enforced jointly by FMCSA and CVSA-certified inspectors at roadside stops.
- Steer tires
The front tires of trucks, tractors, and buses must have at least 4/32 inch of tread remaining in every major groove. The standard is stricter for steer tires than other positions, since the loss of traction while turning on a front tire presents a unique type of danger.
- Drive and trailer tires
Every other position needs at least 2/32 inch.
- CVSA out-of-service criteria
CVSA’s roadside standard sits above the federal minimum in practice. A steer tire measuring less than 2/32 inch triggers an immediate out-of-service order; any other tire measuring less than 1/32 inch does the same. Note the gap: a steer tire at 3/32 inch is already below the FMCSA federal minimum of 4/32 inch and will draw a citation with CSA points, but it sits above the CVSA out-of-service line, so the truck keeps moving, for now. A steer tire at exactly 2/32 inch is below the federal minimum and will be cited, but is not yet at the CVSA OOS threshold.
- Passenger vehicles
NHTSA tire safety guidance identifies 2/32 inch as the point where wear bars become flush with the tread. Inspection requirements vary by jurisdiction, but most states use 2/32 inch as the practical replacement threshold.
When tires should be replaced
The legal minimum and the practical replacement point aren’t the same number, and treating them as interchangeable is where most tire-related violations come from.
| Tire type | Recommended replacement |
| Passenger vehicle | 4/32″ for reliable wet-weather grip |
| Winter tire | 6/32″ |
| Commercial steer tire | Before reaching 4/32″ |
| Drive and trailer tire | Before reaching 2/32″ |
Tread depth is not the only replacement trigger fleets need to track. NHTSA notes that as tires age they become more prone to failure, and both vehicle and tire manufacturers recommend replacing tires after six years of service regardless of remaining tread depth, with a hard limit of ten years from the date of manufacture. The manufacturing date is molded into every tire’s sidewall as the last four digits of the DOT code: the week and year the tire was built. For fleets running vehicles with low annual mileage or irregular use, age can become the binding constraint before tread ever does.
Waiting until a tire hits the legal floor means it’s already performing at its worst for weeks or months beforehand. Replacing at the thresholds above keeps a fleet ahead of both the safety curve and the inspector’s tape measure.
How to measure tire tread depth
Three methods cover nearly every situation a driver or technician runs into.
Tread depth gauge
The only method that gives an exact reading. The probe drops into the groove, and a sliding scale or digital display shows the depth directly. Gauges cost a few dollars and are accurate enough for DOT compliance records.
Wear bar check
Every tire manufacturer molds raised rubber bars into the tread at the 2/32 inch level. Once the tread surface sits flush with these bars, the tire has hit the legal minimum for most positions.
Coin test
The everyday fallback. Place a penny in the smallest groove with Lincoln’s head facing downward. If the head is fully visible, the tread depth is 2/32 inch or less. A similar test can be done with a quarter to estimate the 4/32 inch mark.
Tread should be inspected from the inside, middle, and outside edges of the tire face. Misalignment, improper inflation, and load can cause uneven wear, and checking only one point may miss a worn groove that doesn’t meet the minimum standard. Tires are also a mandatory checkpoint during every pre-trip inspection — catching wear issues before the truck leaves the yard is the first line of defense against a roadside violation.
Why tread depth affects more than compliance
Tread loss changes how a tire interacts with water well before it becomes a legal problem. AAA’s highway-speed testing, conducted with the Automobile Club of Southern California’s Automotive Research Center, found that tires worn to 4/32 inch increased wet stopping distance by an average of 87 feet for a passenger car compared to new tires — a 43% jump. Below 4/32 inch, the grooves no longer have enough depth to channel water out from under the tire fast enough.
Winter conditions push the safe threshold higher still. Snow needs to pack into the tread grooves to create the snow-on-snow contact that grips ice and compacted snow, and that effect drops off once tread falls below 6/32 inch.
Tread wear also shows up on the fuel bill before it shows up on an inspection report. Underinflation and misalignment both accelerate wear and raise rolling resistance long before a tire reaches 2/32 inch. Fleets that only check tires during scheduled maintenance windows are usually the last to catch this.
The limits of manual tread checks
A pre-trip inspection or a scheduled maintenance check is a snapshot. A tire reading 5/32 inch on a Monday morning could be down to 3/32 inch by Friday if there’s a slow leak or an alignment problem wearing one side faster than the rest. A manual check doesn’t catch that trend between two points in time, because nobody’s looking at the tire in between.
That’s the gap continuous monitoring is built to close. Instead of a single reading once a day or once a week, sensor data on pressure, temperature, and wear pattern gets pulled continuously, so a developing problem shows up before it turns into a roadside violation or a blowout.
How Intangles supports tire health monitoring
Intangles is a digital twin company for commercial fleets, and tire and tread monitoring is one part of the predictive vehicle health picture it builds. Its InGenious device connects through a vehicle’s OBD port with no modifications required, streaming pressure, temperature, and wear-pattern data continuously rather than at the pace of a scheduled inspection. That continuous stream feeds Intangles’ predictive AI, which flags tires trending toward a compliance threshold with 96% accuracy — well before a driver or inspector would catch the same trend manually.
| Capability | What it does |
| Continuous tire pressure and wear monitoring | Tracks tread and inflation trends between manual checks instead of relying on a single point-in-time reading |
| Predictive replacement alerts | Flags tires approaching the 4/32 inch or 2/32 inch threshold before they trigger a roadside violation |
| Uneven wear detection | Correlates wear patterns with alignment and inflation data to catch the root cause, not just the symptom |
| Maintenance scheduling integration | Routes flagged tires directly into workshop planning |
| Fleet-wide compliance visibility | Gives maintenance teams one view of tread status across every vehicle and axle position |
Explore the platform or get in touch with our team to learn how Intangles helps US fleets improve tire health, reduce downtime, and strengthen compliance through predictive vehicle monitoring.
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Frequently Asked Questions
What is the legal tire tread depth limit in the US?
For passenger cars, the national standard is 2/32 inch. For commercial steer tires, 49 CFR 393.75 sets the minimum at 4/32 inch, while drive and trailer tires are held to 2/32 inch.
How much tread depth is considered good?
Anything from 6/32 inch onward is safe on wet, dry, or lightly snowy roads. Most safety associations, including AAA, recommend replacing tires once they reach 4/32 inch, well ahead of the legal minimum.
Is 4/32 inch tread depth safe?
It’s legal on all commercial positions except steer tires, but it’s also the point where wet-weather stopping distance has already started to degrade. Most fleets treat 4/32 inch as a replace-soon signal rather than a safe long-term operating point.
What tread depth fails a DOT inspection?
Any tire below 2/32 inch fails outright. CVSA’s roadside criteria go further, placing a vehicle out of service immediately for a steer tire under 2/32 inch or any other tire under 1/32 inch.
Does the penny test actually work?
It gives a rough estimate at the 2/32 inch point, while a quarter gives a rough estimate at the 4/32 inch point. Neither gives an exact measurement in 32nds, which is why a tread depth gauge remains the more reliable instrument.
What happens if a commercial tire fails a roadside inspection?
Under CVSA out-of-service criteria, a steer tire measuring less than 2/32 inch, or any other tire measuring less than 1/32 inch, puts the vehicle out of service until the tire is replaced. These CVSA thresholds sit separately from the FMCSA federal minimums of 4/32 inch for steer tires and 2/32 inch for other positions, so a tire can be below the federal minimum and draw a citation without triggering an immediate OOS order.
How often should fleets check tread depth?
Federal regulations require a tire check during every pre-trip and post-trip inspection. Many fleets pair this with continuous monitoring systems so tread and pressure data is available between scheduled checks as well.
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