Tread depth gauges: how to measure tire wear, read the wear bars and spot uneven wear
Measure every groove, log the lowest reading and learn what wear patterns reveal
Safety Behind the Wheel Foundation built this guide from National Highway Traffic Safety Administration (NHTSA) tire guidance and research, federal tire and truck rules, the UK Driver and Vehicle Standards Agency, Occupational Safety and Health Administration (OSHA) guidance and published studies. Every source is listed at the end.
Why tiny differences in tread depth matter
Tread is the pattern of ribs and grooves on the part of the tire that meets the road. The grooves give water and slush a place to go, so the rubber can keep its hold. NHTSA notes that tread matters most on wet or icy roads [1].
NHTSA reports that 511 people died in tire related collisions in 2024 [1]. Worn tread shows up in crash research, too. NHTSA studied a national sample of collisions from 2005 to 2007. In about 9% of them, at least one vehicle had a tire problem just before the collision, such as a blowout, a flat or a failing tire [2]. About 26% of tires worn to 2/32 inch or less were on vehicles with a pre crash tire problem, compared with 8% of tires at 3/32 to 4/32 inch [2].
A newer study looked at Alabama crashes from 2016 to 2020 that involved a vehicle defect. Brake and tire defects made up about 65% of those defects, and improper tread depth was linked to more serious injuries [3].
Tread wears slowly, so most drivers never notice. A gauge gives you a number that is hard to ignore. This guide goes deeper than the tire section of our vehicle maintenance for safety guide.
What a tread depth gauge does, and how it compares
A tread depth gauge measures how deep the grooves are, from the top of the tread to the groove floor. Its job is accident prevention. It tells you a tire is losing grip before that tire fails you in the rain.
In the U.S., most gauges read in 32nds of an inch, and many also show millimeters. Each step on the inch scale is a little less than 0.8 mm.
Comparison compiled by Safety Behind the Wheel Foundation
Wear bars, coin tests and gauges compared
| Tool | How it works | Best use |
|---|---|---|
| Built in wear bars | Raised rubber bridges in the main grooves that show when tread is down to 1/16 inch, which equals 2/32 [4] | A quick check of the bare minimum |
| Coin test | A coin dropped into a groove, as NHTSA describes with a penny [1] | A rough check when no gauge is handy |
| Tread depth gauge | A probe that reads the depth in 32nds or millimeters | Tracking wear and comparing grooves and tires |
Under FMVSS 139, the federal standard for new radial tires on light vehicles, each tire must have at least six wear indicators spaced evenly around it. Tires for rims 12 inches or smaller need at least three [4]. A wear bar only tells you that you have reached the floor. A gauge shows how fast you are getting there and whether one part of the tire is wearing faster than the rest.
Drivers, parents of teen drivers, used car shoppers, tire technicians and fleet managers all use them.
Choosing and buying a gauge
You can find tread depth gauges at auto parts stores, tire shops, repair shops, the auto aisle of general stores and online. A basic model does the job for most households.
- Probe gauges look like a short pen with a sliding pin and a printed scale. They need no battery.
- Dial gauges show the reading with a needle. They are easy to read but bulkier.
- Digital gauges show the number on a screen and can often switch units or store readings. They need batteries.
- Color band gauges add green, yellow and red zones. Check where each band starts so it matches your replacement plan.
How to measure tread depth, step by step
Measure in the main grooves. These are the deep channels that run all the way around the tire and hold the wear bars. British inspectors call them primary grooves and check depth across the central three quarters of the tread and all the way around the tire [5].
Some grooves have raised tie bars or humps in them. U.S. truck rules say not to measure on those spots [6]. A bump stops the pin short of the groove floor, so the reading comes out wrong.
- Park on level ground, set the parking brake and turn the engine off.
- Turn the steering wheel fully to one side so you can reach the whole width of the front tires.
- Brush stones and dirt out of the grooves.
- Check that the gauge reads zero when its pin is flush with its base.
- Set the base flat across the ribs on each side of a main groove. Push the pin straight down to the groove floor.
- Read the gauge without tilting it. Measure the inner, center and outer main grooves.
- Roll the vehicle forward about a third of a wheel turn and measure again. Repeat once more.
- Write down the lowest reading for each tire, with the date and mileage.
- Repeat for every tire, including a full size spare.
The lowest reading matters most. NHTSA crash researchers record tread depth on the shallowest groove of each tire [2].
Check every tire, not just one or two. In an NHTSA survey of vehicles in February 2001, 9% had at least one bald tire, at 2/32 inch or less [7]. Fewer than 1.5% had tires that averaged that low [7]. One worn tire can hide among three good ones.
Reading the number: 32nds, millimeters and your replacement line
NHTSA says tires are no longer safe and should be replaced at 2/32 inch [1]. That is the same height as the wear bars [4]. A 2006 survey of state laws found that most states used 2/32 inch as their minimum, while a few allowed less or had no rule [8]. Check your state’s current rules with your DMV.
The legal floor is not a safe margin. A research commentary in Traffic Injury Prevention explained that on dry, clean roads, grip depends mostly on the road surface, not the tread. On wet roads, water depth, speed, tread depth and road texture all matter [8]. The authors cited research suggesting that tires worn below 4/32 inch may lose about half their grip at highway speed on a lightly wet road, even before hydroplaning [8]. Hydroplaning is when a tire rides up on a film of water and loses contact with the road.
Comparison compiled by Safety Behind the Wheel Foundation
What your gauge reading means
| Gauge reading | Metric | What it means |
|---|---|---|
| 2/32 in | about 1.6 mm | Level with wear bars. NHTSA says replace [1, 4] |
| 3/32 in | about 2.4 mm | Close to the floor; replace soon |
| 4/32 in | about 3.2 mm | Wet grip falls off below here [8]. Minimum for front tires on trucks and buses [6] |
| 5/32 in and up | about 4 mm and up | Keep measuring monthly and watch the trend |
For comparison, cars in Great Britain must keep at least 1.6 mm in the primary grooves [5]. That equals 2/32 inch.
Track the trend
One reading shows where a tire is today. A log shows how fast it is wearing. If one tire loses tread faster than the others, look for a cause.
New tires carry a treadwear grade on the sidewall. NHTSA explains that a tire graded 200 should last about twice as long as one graded 100 [9]. Your real wear rate still depends on pressure, alignment, load and driving.
What uneven wear is telling you
A healthy tire wears at about the same rate across its width. When one part wears faster, that part is working harder than it should. NHTSA lists irregular wear from low pressure or overloading as a reason to take a tire out of service [1]. It also notes that balancing keeps wheels from shaking, alignment keeps the car from drifting and extends tire life, and rotation reduces irregular wear [1].
The table shows common patterns. A technician should confirm the cause before you pay for a fix.
Comparison compiled by Safety Behind the Wheel Foundation
Uneven wear patterns and what they point to
| What you see | What it often points to | What to ask for |
|---|---|---|
| Both outer edges worn more than the center | Low pressure or heavy loads | Pressure check, load review |
| Center worn more than both edges | Pressure set above the door label | Reset to the door label pressure |
| One edge worn, inner or outer | Alignment off, or bent parts after a curb or pothole hit | Alignment and suspension inspection |
| Ribs smooth one way, sharp the other | Feathering, often from alignment settings | Alignment check |
| Scalloped dips around the tire | Cupping, often from worn shocks or struts or an unbalanced wheel | Suspension and balance check |
| One bald patch | A skid with locked wheels, or a brake problem | Brake inspection and tire check |
| One tire far more worn than the rest | Rotation overdue, or a problem at that wheel | Rotation and an inspection of that corner |
To feel feathering, slide your palm across the tread one way, then the other. Smooth one way and rough the other means the ribs are being scrubbed sideways.
Inner edge wear is easy to miss because you cannot see it from the curb. That is why step 2 above matters. If the inner groove reads clearly lower than the outer groove, have the alignment checked.
Low pressure and worn tread often go together. In NHTSA’s 2001 survey, tires with less tread were more often low on air [7]. Check pressure every time you check tread.
If wear starts soon after a pothole or curb strike, see when your car pulls or shakes after a crash.
Damage a gauge cannot measure
A tire can have plenty of tread and still be unsafe. NHTSA lists cuts, cracks and bulges as signs a tire should come off the road [1]. It also advises checking for cuts and scrapes, and having tires checked each year for rubber breakdown, which sun and heat speed up [10].
A bulge or lump usually means the tire’s inner structure is coming apart. British inspectors treat a bulge from this kind of failure as a dangerous defect [5]. U.S. truck rules bar tires that show belt or ply cords, have tread or sidewall separation, leak audibly or have a cut that exposes the cords [6]. Treat those signs the same way on a family car.
If you find a nail or screw, leave it in place, since pulling it out can speed up the leak. Drive gently to a tire shop and ask whether a proper repair is possible.
Age matters too. The last four digits of the DOT Tire Identification Number on the sidewall tell you when the tire was built, as a week and a year [9]. Some vehicle and tire makers advise retiring tires once they are six to ten years old, whatever the tread depth [1].
Before rain, snow and long trips
NHTSA advises a tire check every month and before any long drive [11]. Safety Behind the Wheel Foundation gives drivers the same advice. NHTSA also suggests looking at the tread whenever you check pressure or rotate tires [12].
Before the rainy season. In NHTSA’s crash study, tire problems were more common when the road was wet, flooded or slick. In those collisions, 11.2% of vehicles had one [2]. If your lowest reading is near 4/32 inch, replace the tires before the rains, not after. For driving tactics, see our guide to driving in rain, hydroplaning and flooding.
Before the first snow. Snow and slush also need open grooves. Measure in the fall, while you still have time to shop. Cold weather lowers tire pressure, so check pressure as it gets colder [11]. A companion guide covers dedicated winter tires. For driving skills, see driving in snow and ice.
Before a road trip. A loaded car strains every tire, and NHTSA links overloading to irregular wear and damage [1]. Measure each tire, set the pressure from the door label and look closely at the sidewalls.
Tread checks in a fleet inspection program
Work vehicles pile up miles fast. OSHA urges employers to set up a preventive maintenance program, with regular scheduled checks, so work vehicles stay safe [13].
Federal rules for commercial trucks and buses set firm lines. Front tires need at least 4/32 inch in every major groove, and other tires need at least 2/32 inch [6]. Fleets that run cars, vans and pickups can use these numbers as a sensible internal guide.
Vehicle type matters. In NHTSA’s crash study, 45% of SUVs that had a tire problem before a collision rolled over [2]. For cars, pickups and vans, the share was under 25% [2].
If the log shows one position wearing fast across several vehicles, check alignment and loading habits. For more, see our guide to safer fleet vehicles.
Common mistakes and how to care for your gauge
Watch for these common errors.
- Measuring on a wear bar or tie bar. The bump gives a false reading.
- Checking only the outer groove. This misses inner edge wear.
- Taking one reading per tire. Wear can vary around the tire.
- Tilting the gauge. A base that is not flat gives a false number.
- Mixing up the scales. Read 32nds or millimeters, not a blend.
- Trusting depth alone. Age, cuts, bulges and uneven wear can end a tire’s life early [1].
A bad gauge gives bad numbers. Keep it clean and dry, and confirm it reads zero before each use. Replace a gauge with a bent pin, a sticky slide or a cracked base. For digital models, keep a spare battery in the glove box.
Frequently asked questions
How often should I measure my tread?
Measure at least once a month, when you check tire pressure, and before long trips [11, 12]. Add a check before the rainy season and before winter. If your log shows fast wear, measure more often.
Is a gauge better than the penny test?
A coin gives a rough answer at one spot. NHTSA describes the penny test as a way to see whether tread has reached the 2/32 inch floor [1]. A gauge gives an exact number in every groove, so you can spot uneven wear and plan ahead.
At what reading should I replace my tires?
NHTSA says to replace tires at 2/32 inch [1]. Research suggests wet road grip drops off below about 4/32 inch [8], so that is a sensible time to start shopping. Front tires on commercial trucks and buses must have at least 4/32 inch [6]. Also check your vehicle maker’s advice and your state’s rules.
Why do my front tires wear faster than my rear tires?
On many vehicles, the front tires steer and do much of the braking. On front wheel drive cars, they also put the engine’s power to the road. That extra work wears them faster. Regular rotation helps even out wear [1]. Your owner’s manual gives the rotation schedule for your vehicle.
What should I do if one tire reads much lower than the others?
Measure again to rule out a bad reading, then check that tire’s pressure and look for damage. One fast wearing tire often points to a problem at that wheel, such as alignment, a dragging brake or worn suspension parts. Have that corner inspected before you replace the tire.
The bottom line
Recommendation from Safety Behind the Wheel Foundation
Safety Behind the Wheel Foundation recommends checking tread depth on every tire, not just glancing at the wear bars. A tread depth gauge is one of the cheapest safety tools you can own. Used monthly, it shows how much grip your tires have left, which tire is wearing fastest and whether something under the car needs attention.
Measure every groove on every tire, log the lowest reading and treat 2/32 inch as the floor, not the goal. Read the wear pattern as closely as the number. Those few minutes can keep a worn tire from turning a rainy commute into a wreck.
Related articles
- Vehicle maintenance for safety: tires, brakes, lights and warning signs
- Driving in rain: hydroplaning, standing water and flood safety
- Driving in snow and ice: grip, black ice, skids and when to stay home
- When your car pulls or shakes after a crash
- Fleet vehicle safety: how to choose, maintain and inspect work vehicles
Sources and further reading
- 1National Highway Traffic Safety Administration (NHTSA). Tire Safety Ratings and Awareness (TireWise). Updated: not stated; accessed October 2026. Data year: 2024 (tire-related crash deaths); n/a (guidance). nhtsa.gov/vehicle-safety/tires
- 2NHTSA, National Center for Statistics and Analysis. Tire-Related Factors in the Pre-Crash Phase (DOT HS 811 617). Published April 2012. Data year: 2005 to 2007 (National Motor Vehicle Crash Causation Survey). crashstats.nhtsa.dot.gov/…/811617
- 3Adanu EK, Dzinyela R, Okafor S, Jones S. Injury-severity analysis of crashes involving defective vehicles and accounting for the underlying socioeconomic mediators. Heliyon 10(5):e26944 (PubMed record). Published February 2024. Data year: 2016 to 2020 (Alabama). [https://pubmed.ncbi.nlm.nih.gov/38434351/](https://pubmed.ncbi.nlm.nih.gov/38434351/) (DOI: [https://doi.org/10.1016/j.heliyon.2024.e26944](https://doi.org/10.1016/j.heliyon.2024.e26944)) doi.org/10.1016/j.heliyon.2024.e26944
- 4Electronic Code of Federal Regulations. 49 CFR 571.139, Standard No. 139; New pneumatic radial tires for light vehicles (S3 and S5.4, treadwear indicators). Up to date as of October 1, 2026. ecfr.gov/…/section-571.139
- 5Driver and Vehicle Standards Agency (UK). MOT inspection manual: cars and passenger vehicles. 5. Axles, wheels, tyres and suspension. Updated June 1, 2026. gov.uk/…/5-axles-wheels-tyres-and-suspension
- 6Electronic Code of Federal Regulations. 49 CFR 393.75, Tires (Federal Motor Carrier Safety Regulations). Up to date as of October 2, 2026. ecfr.gov/…/section-393.75
- 7NHTSA. Tire Pressure Special Study: Tread Depth Analysis (DOT HS 809 359). Published October 2001. Data year: 2001 (vehicle survey, February 1-14, 2001). crashstats.nhtsa.dot.gov/…/809359
- 8Blythe W, Seguin DE. Commentary: legal minimum tread depth for passenger car tires in the U.S.A.–a survey. Traffic Injury Prevention 7(2):107-110 (PubMed record). Published June 2006. [https://pubmed.ncbi.nlm.nih.gov/16854703/](https://pubmed.ncbi.nlm.nih.gov/16854703/) (DOI: [https://doi.org/10.1080/15389580500500640](https://doi.org/10.1080/15389580500500640)) doi.org/10.1080/15389580500500640
- 9NHTSA. Tire Buyers’ FAQ: What You Should Know and Ask (TireWise handout). Updated: not stated; accessed October 2026. nhtsa.gov/node/35301
- 10NHTSA. Tires in the Garage (TireWise infographic). Updated: not stated; accessed October 2026. nhtsa.gov/node/35311
- 11NHTSA. Winter Weather Driving Tips: Prepare Your Vehicle. Updated: not stated; accessed October 2026. nhtsa.gov/winter-driving-tips
- 12NHTSA. Congratulations on Your New Tire Purchase (TireWise handout). Updated: not stated; accessed October 2026. nhtsa.gov/node/35306
- 13Occupational Safety and Health Administration (OSHA). Motor Vehicle Safety for Employers (fact sheet). Date not stated; accessed October 2026. osha.gov/…/Motor-Vehicle-Safety_FactSheet_Employe…
This article reflects the safety priorities of tire care and everyday vehicle maintenance. It is general education, not legal, insurance or mechanical advice for your vehicle. Rules change, so verify details with official sources. Follow your owner's manual and the tire maker's guidance, and ask a qualified tire or repair shop to inspect any tire with uneven wear or damage. Fleet operators should check current FMCSA and state rules. Reviewed October 2026.
Cite this article: Safety Behind the Wheel Foundation. “Tread depth gauges: how to measure tire wear, read the wear bars and spot uneven wear.” drivewithcare.org, https://drivewithcare.org/tread-depth-gauge-tire-wear/. Reviewed October 2026.
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