Maintenance procedure · Written to ASD-STE100 Issue 9 · Formatted to ANSI Z535

Check Tire Tread Depth: Wear Bars, the Penny Test, and a Tread Depth Gauge

Passenger car and light truck tires — find the treadwear indicators, measure every groove, and read the wear pattern
ApplicabilityPassenger car and light truck tires on a vehicle parked on level ground
Task time18 min editorial estimate, not a measurement
PersonnelOne
Revisionv1 · 2026-07-29
BEFORE YOU BEGIN

Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.

Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.

Summary

Every tire sold in the United States carries its own wear gauge, moulded into the tread. Federal Motor Vehicle Safety Standard No. 139 requires at least six treadwear indicators — almost everyone calls them wear bars — spaced around the tire, and they become level with the tread surface when the tread has worn to one sixteenth of an inch. NHTSA writes the same depth in the other unit and says a tire is not safe and should be replaced when the tread is worn down to 2/32 of an inch. This page shows you where the bars are, three ways to read the depth, and what an uneven wear pattern is telling you.

It does not print a legal minimum for your car. The federal figures that do exist — 4/32 of an inch on the front wheels of a bus, truck or truck tractor, and 2/32 of an inch elsewhere — belong to the FMCSA rule for commercial vehicles in interstate commerce, not to a private car, and for a private car the minimum is state law and it varies. It also does not print an early-replacement depth for wet weather, because no primary source publishes one for passenger cars and a number invented to fill the gap would be worse than the gap. Your vehicle documentation and your tire manufacturer are the authority for your own tires, and this page tells you what to measure before you ask them.

Necessary equipment

ItemNote
Tread depth gauge optionalUse a tread depth gauge to get a number for each groove.
United States penny optionalUse a penny if you do not have a tread depth gauge.
Flashlight optionalThe light near a tire is often low.
ClothUse a cloth to remove dirt from the tread.
Pen and paperRecord the measurements for each tire.

Specifications

Tread depth required on the front wheels of a bus, truck or truck tractor in interstate commerce
Manufacturer or standard verified
4/32 in

Source: 49 CFR 393.75 — Tires · tier 1 · accessed 2026-07-29

Tread depth required on the other wheels of a commercial vehicle in interstate commerce
Manufacturer or standard verified
2/32 in

Source: 49 CFR 393.75 — Tires · tier 1 · accessed 2026-07-29

Tread depth at which to replace a winter tire or an all-season tire early
Unverified — do not rely on this value
CONSULT THE TIRE MANUFACTURER AND THE VEHICLE DOCUMENTATION

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

Prepare the vehicle

Estimated 2 min

WARNING

STOP THE VEHICLE AWAY FROM TRAFFIC BEFORE YOU START.
Traffic can kill you.

  1. Stop the vehicle on a level surface away from traffic.
    <1m
  2. Set the parking brake.
    <1m
  3. If you examine a front tire, turn the steering wheel fully to one side.
    • More of the tread of a front tire is then in view.
    <1m
  4. If the light is low, use a flashlight.
    <1m
WARNING

DO NOT PUT YOUR FINGERS INTO A CUT IN THE TREAD.
A nail or a piece of glass in the tread can cut you.

  1. Use a cloth to remove dirt from the tread.
    <1m

Find the treadwear indicators

Estimated 2 min

NOTEThe treadwear indicators are in the bottom of the grooves.
Least number of treadwear indicators on a tire with a rim diameter above 12 inches
Manufacturer or standard verified
6

Source: 49 CFR 571.139 — Standard No. 139; New pneumatic radial tires for light vehicles · tier 1 · accessed 2026-07-29

  1. Look at the tread of one tire.
    <1m
  2. Find the grooves between the tread ribs.
    • A tread rib is a part of the tread that goes around the tire.
    • A groove is the space between two tread ribs.
    <1m
  3. Find a raised bar in the bottom of a groove.
    • The federal standard calls this bar a treadwear indicator.
    • A tire with a rim diameter of more than 12 inches has six or more treadwear indicators.
    • The treadwear indicators are at approximately equal distances around the tire.
    <1m
  4. If you do not find a bar, look in a different groove.
    <1m

Measure the tread depth

Estimated 10 min

NOTEMeasure each principal groove of each tire.
Tread depth at which a treadwear indicator becomes level with the tread
Manufacturer or standard verified
1/16 in

Source: 49 CFR 571.139 — Standard No. 139; New pneumatic radial tires for light vehicles · tier 1 · accessed 2026-07-29

Same depth as the metric figure the federal standard prints
Manufacturer or standard verified
1.6 mm

Source: 49 CFR 571.109 — Standard No. 109; New pneumatic tires · tier 1 · accessed 2026-07-29

Tread depth at which NHTSA states a tire is not safe
Manufacturer or standard verified
2/32 in

Source: Tire Safety Ratings and Awareness (TireWise) · tier 1 · accessed 2026-07-29

Minimum tread depth the law requires for a private car where you drive
Unverified — do not rely on this value
CONSULT THE LAW OF THE STATE OR THE COUNTRY WHERE YOU DRIVE

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

Tread depth at which your vehicle manufacturer says to replace the tire
Unverified — do not rely on this value
CONSULT THE VEHICLE DOCUMENTATION

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

Tread depth of your tire when it was new
Unverified — do not rely on this value
CONSULT THE TIRE MANUFACTURER

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

  1. Look at the top of a treadwear indicator.
    <1m
  2. Compare the top of the indicator with the surface of the tread rib.
    <1m
WARNING

IF THE TREAD IS LEVEL WITH A TREADWEAR INDICATOR, DO NOT CONTINUE TO USE THE TIRE.
A worn tire increases the distance to stop and can cause a crash that injures you.

  1. If the tread is level with the indicator, the tread depth is one sixteenth of an inch.
    • One sixteenth of an inch is 1.6 millimetres.
    • Some documents write this same depth as two thirty-seconds of an inch.
    <1m
NOTICE

DO NOT MEASURE AT A TIE BAR OR AT A HUMP IN A GROOVE.
The result there is smaller than the true tread depth.

  1. If you have a tread depth gauge, put the pin of the gauge into a groove.
    <1m
  2. Push the foot of the gauge flat onto the tread ribs.
    <1m
  3. Read the number on the gauge.
    <1m
  4. Measure each principal groove of the tire.
    • The grooves do not always wear at the same rate.
    1m
  5. Keep the smallest of the numbers for this tire.
    <1m
  6. If you do not have a gauge, use a United States penny.
    <1m
  7. Turn the penny until the head of Lincoln points into the groove.
    <1m
  8. Put the penny into a groove of the tread.
    <1m
  9. If you see the top of the head, the tread is at the limit or below it.
    • NHTSA tells drivers to replace a tire when the top of the head is in view.
    <1m
NOTICE

Do not use one measurement for all the tires.
The tires on one vehicle wear at different rates.

  1. Do the same measurements for each tire on the vehicle.
    5m

Examine the wear pattern

Estimated 3 min

  1. Look at the tread across the full width of the tire.
    <1m
  2. Compare the tread depth at the two outer edges and at the center.
    <1m
  3. If the tread depth at the center is less than at the two edges, record this result.
    <1m
  4. If the tread depth at the two edges is less than at the center, record this result.
    <1m
  5. If the tread depth at one edge is less than at all other points, record this result.
    <1m
  6. Examine the tread and the sidewall for cuts, cracks and bulges.
    <1m
  7. If you find a cut, a crack or a bulge, speak to a tire specialist.
    <1m

Record the result

Estimated 2 min

NOTEA dated record shows how quickly the tread wears.
How often NHTSA states to look at the tread
Manufacturer or standard verified
1 month

Source: Tire Safety Ratings and Awareness (TireWise) · tier 1 · accessed 2026-07-29

  1. Record the smallest tread depth for each tire.
    <1m
  2. Record the date of the measurements.
    <1m
  3. Keep the record in the vehicle.
    <1m
  4. Do this procedure one time each month.
    <1m

Terminology

This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.

treadwear indicator
Also called: wear bar, wear bars, tread wear indicator, TWI, the little bars in the tire grooves, tire wear indicator
tread depth
Also called: tire tread, tyre tread depth, how much tread is left, tread left on my tires, tire tread wear
tread depth gauge
Also called: tire tread gauge, tread meter, tread depth tool, tire depth gauge
penny test
Also called: coin test, quarter test, Lincoln test, penny trick, penny in the tire
groove
Also called: tread groove, principal groove, major groove, channel, the gaps in the tire
tread rib
Also called: tread block, the raised part of the tread, tread pattern
uneven wear
Also called: irregular wear, abnormal wear, tires wearing on one side, cupping, feathering, worn on the inside edge

When to use a professional

Stop if the tread is level with a treadwear indicator. Stop if you see cord or belt material through the tread. Stop if the tire has a cut, a crack or a bulge. Speak to a tire specialist.

A tire specialist must tell you if a tire is safe to use. This page does not give a legal minimum for a private car. The law of your state gives that number. Read the documentation for your vehicle and for your tires. Speak to a tire specialist about uneven wear, because uneven wear can show a fault in a different part.

Common questions

How do I check my tire tread depth?

There are three ways, in order of how exact they are. A tread depth gauge gives you a number for every groove and costs very little. The treadwear indicators moulded into the grooves tell you when the tread has reached one sixteenth of an inch, which is the depth the federal standard makes them mark. A United States penny, held so that Lincoln's head points down into a groove, is the rough version: NHTSA's instruction is to replace the tire if the top of his head stays in view. Measure every groove on every tire, because tires rarely wear evenly and the shallowest groove is the one that counts.

What are the little bars in my tire grooves?

They are treadwear indicators, and almost everyone calls them wear bars. Federal Motor Vehicle Safety Standard No. 139 requires at least six of them, at roughly equal spacing around the tire, on a radial tire for a light vehicle with a rim diameter over 12 inches. Deep-tread light truck tires — 18/32 of an inch or more when new — sit outside FMVSS 139 and under FMVSS 119 instead, which asks for the same six indicators at the same depth, so the answer is the same whichever standard your tire was built to. They sit in the bottom of the principal grooves at a height that lets you see, without any tool, whether the tread has worn to one sixteenth of an inch. Mazda's 2026 Mazda3 owner's manual makes the same point without printing a number: on a tire that has worn evenly the indicators join into one solid band across the tread, and that band is the signal to replace the tire.

Does the penny test actually work?

It works as a yes-or-no test, not as a measurement. NHTSA's version is to put a penny into a groove upside down and head first, and to replace the tire if the top of Lincoln's head remains in view. What it will not tell you is how much tread you have left, which groove is worst, or how fast you are losing it. A tread depth gauge costs a few dollars and answers all three. If the only question is whether you need tires today, the penny is enough.

Is 2/32 of an inch the legal minimum tread depth?

Not federally, and not for a private car. The federal rule that prints a number is FMCSA's and it covers commercial vehicles in interstate commerce: 49 CFR 393.75 requires at least 4/32 of an inch on the front wheels of a bus, truck or truck tractor, and at least 2/32 of an inch everywhere else, measured in a major groove and never at a tie bar, hump or fillet. For your own car the minimum is set by state law, it is not the same in every state, and this page will not print a figure for it. What NHTSA does say, for everyone, is that tires are not safe and should be replaced when the tread is worn down to 2/32 of an inch — the same depth the treadwear indicators mark. Look up your own state's rule, and treat that depth as the floor rather than the target.

When do I need new tires?

Tread level with the treadwear indicators is the clearest answer, and NHTSA's position is that a tire worn to 2/32 of an inch is not safe. Depth is not the only trigger. NHTSA also names physical damage such as cuts, cracks and bulges, and irregular wear caused by underinflation or overloading, as reasons to stop using a tire. Age is a separate question again: NHTSA notes that some vehicle and tire manufacturers recommend replacing tires that are six to 10 years old regardless of treadwear, and Mazda's 2026 Mazda3 manual gives six years as its own general recommendation. Our page on reading a tire sidewall shows you how to find the date code so you can answer the age question for your own tires. Your vehicle documentation and your tire maker are the authority, and this page is what you measure before you ask them.

Should I replace my tires earlier than the wear bars for rain or snow?

This page will not print an early-replacement number, because no primary source publishes one for passenger cars. Tread depth and water are genuinely linked: NHTSA describes tread as what gives a car its grip and traction, and singles out wet and icy roads as where that matters most. That is a statement about physics, though, not a second threshold, and turning it into a number would mean inventing one. Tire manufacturers publish their own advice for winter and all-season products, and they are the people to ask about a specific tire. If you drive on wet roads a lot, the honest move is to measure monthly and watch the trend rather than wait for the bars to arrive.

Why is one part of my tire more worn than the rest?

Uneven wear is a symptom, and the cause is usually somewhere other than the tire. Mazda's 2026 Mazda3 owner's manual lists the usual causes of abnormal wear as incorrect tire pressure, improper wheel alignment, a wheel that is out of balance, and severe braking. Wear concentrated in the centre, or at both shoulders, points at inflation. Wear on one shoulder only points at alignment. This page will not tell you which one it is, because measuring is a diagnostic and not a repair, so take the measurements with you to a tire specialist. Set the pressure from the vehicle placard first, because that costs nothing and it is the most common cause.

How often should I check tread depth?

NHTSA's guidance is to check the tread at least once a month, at the same time you check the pressure. Make it one trip, because the vehicle has to be parked and cold for a pressure reading anyway. Write the numbers down with the date. One measurement tells you where you are today. A year of them tells you how long the tires have left, which is the thing you actually want to know.

Sources

Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.

  1. Tier 1Tire Safety Ratings and Awareness (TireWise)National Highway Traffic Safety Administration · accessed 2026-07-29 · re-verification due 2027-01-25 · automated link check returned 403 — verified by hand
  2. Tier 149 CFR 571.139 — Standard No. 139; New pneumatic radial tires for light vehiclesOffice of the Federal Register / National Archives (eCFR), for the National Highway Traffic Safety Administration · accessed 2026-07-29 · re-verification due 2027-01-25
  3. Tier 149 CFR 571.109 — Standard No. 109; New pneumatic tiresOffice of the Federal Register / National Archives (eCFR), for the National Highway Traffic Safety Administration · accessed 2026-07-29 · re-verification due 2027-01-25
  4. Tier 149 CFR 571.119 — Standard No. 119; New pneumatic tires for motor vehicles with a GVWR of more than 4,536 kilograms (10,000 pounds) and motorcyclesOffice of the Federal Register / National Archives (eCFR), for the National Highway Traffic Safety Administration · accessed 2026-07-29 · re-verification due 2027-01-25
  5. Tier 149 CFR 393.75 — TiresOffice of the Federal Register / National Archives (eCFR), for the Federal Motor Carrier Safety Administration · accessed 2026-07-29 · re-verification due 2027-01-25
  6. Tier 1Replacing a Tire or a Wheel — 2026 Mazda3 Owner's ManualMazda North American Operations · accessed 2026-07-29 · re-verification due 2027-01-25
  7. Tier 1Rotating the Tires — 2026 Mazda3 Owner's ManualMazda North American Operations · accessed 2026-07-29 · re-verification due 2027-01-25

Revision history

Current version 1 · updated 2026-07-29 · next review due 2027-07-29

  1. v1 · 2026-07-29Re-gate pass on the same draft; no version bump because the page has never published. Five STE errors cleared by rewriting rather than by justification: three passive 'is worn' constructions in the wear-pattern steps became comparisons of tread depth, and two four-word noun clusters were broken up. Added 49 CFR 571.119 S6.4 as a source and named it in the wear-bar answer, because FMVSS 139 S2 does not apply to light truck tires with a tread depth of 18/32 inch or greater and the applicability line claims light truck tires; FMVSS 119 sets the same six indicators at the same 1.6 mm, so this closes a coverage gap and is not a conflict. Added two related links. Every cited source was re-read and link-checked on this date. Five of the six source ids are not yet in data/sources.json — this session could not write the registry, and the entries are staged in data/sources-additions-check-tire-tread-depth.json for merge. The record holds on checks 2 and 3 until that merge; nothing else fails.
  2. v1 · 2026-07-29First draft. Built from 49 CFR 571.139 S5.4, 49 CFR 571.109 S4.2.1(d), 49 CFR 393.75, NHTSA TireWise and the 2026 Mazda3 owner's manual. The treadwear-indicator depth publishes as a verified value; the legal minimum for a private car, the manufacturer replacement depth and any early-replacement depth for wet weather publish as consult-the-manufacturer.

Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.

Comments and corrections

Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.

Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.

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