Tire tread wear indicators and accurate tread checks

Does a tread wear indicator prove that the whole tire is serviceable?

No. A tread wear indicator is a raised bar in a principal groove that gives a visual threshold at that position; it does not measure every groove or approve the tire’s overall condition. UN R30 specifies 1.6 mm-high indicators for covered new passenger-car tires, while NHTSA and USTMA use 2/32 inch (1.6 mm) in their U.S. consumer guidance. Measure with a tread-depth gauge at several points across and around every tire, record the lowest relevant reading, inspect for damage and uneven wear, and apply the vehicle/tire instructions and current law where the vehicle is used. Damage or abnormal behavior can require removal before a wear bar is flush.

What a TWI bar does and does not show

Sidewall triangles, arrows or TWI lettering commonly point toward narrow raised bars across principal grooves. When surrounding tread is level with a bar, that location has reached the bar height. Indicator count, placement and any secondary winter or product-specific markers vary by tire, so identify them from the exact tire documentation rather than assuming six, eight or one universal secondary depth.

The bar is a visual threshold, not a calibrated reading above it. It cannot reveal the shallowest point elsewhere, internal damage, age, pressure history, a legal winter requirement or remaining wet-grip performance. A tire can need removal even when every bar is still below the tread surface.

Repeatable tread inspection workflow

Step, What to do, What to record
Step	What to do	What to record
Prepare safely	Park on a level safe surface, secure the vehicle and turn the steering only as the vehicle instructions allow access. Use adequate light; never work under an unsupported vehicle.	Tire position, date, odometer and any warning, vibration, pull or pressure loss.
Inspect condition	Check accessible tread and sidewalls around the circumference for cuts, cracks, bulges, exposed material, foreign objects and localized deformation.	Location and type of every abnormality; do not remove a deeply embedded object just to continue driving.
Locate indicators	Follow the exact sidewall marks into the principal grooves and compare the surrounding tread with each bar.	Which bars are closest to flush and whether wear differs around the tire or across its width.
Measure	Place a suitable tread-depth gauge squarely in the grooves specified by the applicable rule or inspection procedure. Repeat across the tread and at several circumferential positions.	Every reading and the lowest relevant value; keep units explicit and do not round upward.
Decide	Compare the result with the strictest applicable vehicle, tire, inspection, seasonal and local requirement. Escalate damage or irregular wear to a qualified tire professional.	Controlling source, decision and any corrective inspection for pressure, alignment, balance, wheel, brake or suspension.

Gauge first; coin tests are only rough prompts

A suitable tread-depth gauge gives a numeric reading and can expose variation that a wear bar or coin cannot. Clean loose debris, keep the gauge base flat, avoid raised bars and repeat any surprising result. A named coin test depends on one coin design and one orientation; it is neither a calibrated measurement nor a universal legal test and must not replace a gauge or a prescribed inspection method.

Uneven wear is evidence, not a one-cause diagnosis

Observation, Possible contributors, Next check
Observation	Possible contributors	Next check
Both shoulders lower	Pressure/load history, cornering, axle duty, road and tire construction.	Verify cold placard pressure, load and service history; inspect for heat or structural damage.
Centre lower	Pressure/load relationship, rim/tire combination, speed and duty cycle.	Confirm approved rim, placard pressure and operating load; do not correct from shape alone.
One shoulder or feathering	Alignment geometry, steering/suspension wear, wheel damage or axle condition.	Measure alignment and inspect steering, suspension and wheel condition.
Cupping or diagonal patches	Balance, runout, damping, suspension, wheel or brake condition.	Have the wheel/tire assembly and related vehicle systems diagnosed.
Localized flat or distorted area	Braking event, storage, impact, separation or other damage.	Stop relying on tread depth alone and obtain a qualified tire inspection.

Why there is no worldwide legal-limit table here

A 1.6 mm wear bar is not proof that one legal rule applies to every country, axle, tire category, vehicle class or season. ETRTO explicitly notes that minimum tread requirements can vary by country and that no single remaining depth defines safety for every tire and condition. Check the current rule for the place of use and the exact inspection scope; penalty and insurance outcomes are jurisdiction- and case-specific.

Replace or stop for inspection when

the lowest relevant measurement is at or below the applicable limit, or a required wear indicator is flush;
cord or fabric is exposed, or there is a bulge, separation sign, serious cut, crack or deformation;
pressure loss, vibration, pulling, noise or a damaging impact suggests a condition not resolved by a tread measurement;
the vehicle or tire maker requires earlier removal for the exact product, service or seasonal use;
irregular wear cannot be explained and corrected before replacement.

Do not use these shortcuts

one centre-groove reading as proof for the complete tire;
a visible gap above one TWI bar as a serviceability approval;
fixed wet-braking percentages or an aquaplaning speed from tread depth alone;
a coin as a calibrated gauge or universal legal test;
one 3 mm, 4 mm, winter or penalty rule for every market;
one wear pattern as proof of one mechanical fault.

Continue the inspection

Limits and safety boundary

This guide supports screening and record keeping; it cannot inspect hidden construction, certify roadworthiness, predict stopping distance or state the law for every jurisdiction. Where the condition or requirement is uncertain, stop using the shortcut and obtain the exact rule or a qualified physical inspection.

Reviewed sources

These sources support TWI design context, U.S. consumer replacement guidance, multi-factor inspection and the limits of tread-depth-only conclusions. They do not support universal braking percentages, coin accuracy, penalty tables or one worldwide replacement depth.

Checked: 2026-08-21

Last reviewed: 2026-08-21