Tire Tread Depth: Measurement, Wear Bars and Wet Grip
Tread depth is the vertical distance from a tire groove’s base to the tread surface. It should be measured in several primary grooves and at multiple positions around each tire because wear can be uneven. Wear bars provide a quick replacement signal, while a gauge gives a more precise reading.
Timeline
- 2026-10-13: U.S. NHTSA and UK government tread, wear-indicator and tire-condition guidance was reviewed for this guide.
- 2026-10-13: The current authoritative sources cited in this guide were reviewed.
Tire tread depth measures the remaining height of the tread above the bottom of a main groove. Those grooves provide edges for grip and space for water to escape from under the contact patch. NHTSA says passenger-vehicle tires should be replaced when tread reaches 2/32 inch, about 1.6 millimetres, and emphasizes traction on wet or icy roads. That number is a U.S. safety recommendation in this context, not a worldwide legal rule. Vehicle type and jurisdiction control the enforceable minimum, and conditions may justify replacement before the floor is reached. [1][2]
Measure with a tread-depth gauge in several primary grooves across the tire and at several positions around its circumference. Park safely, set the brake, and turn the steering only if it can be done without creating a hazard. Seat the gauge base flat on the tread and extend its probe to the bottom of a main groove, avoiding a raised wear bar. Record the lowest valid reading. The UK inspection manual distinguishes primary grooves from shallow secondary grooves and tie-bars, showing why measuring a decorative sipe or the top of an indicator can give the wrong answer. [3]
Built-in treadwear indicators are raised bars molded across selected grooves. When the surrounding tread becomes level with a bar, NHTSA says the tire has reached replacement depth. Federal test procedures place these indicators around 1.6 millimetres, subject to the standard’s tolerance. A bar is a threshold marker rather than a continuous gauge: it cannot show how much tread remains above it, and one bar may miss wear elsewhere. The sidewall often has small marks identifying indicator locations, but the tire maker’s documentation and a gauge are more reliable than guessing from appearance. [1][4]
A coin test is a screening tool. NHTSA’s U.S. penny method places Lincoln’s head upside down in a groove; if the top of the head is visible, replacement is due. Other countries publish different coin checks based on their currency. A coin is less precise than a calibrated gauge, can be tilted, and samples only one location. Use it to identify an obvious problem, then check all tires and multiple grooves. A tire that passes at the center can still be unsafe or unlawful if a shoulder, inner edge or section around the circumference is worn or damaged. [1][5]
Legal minima must be read exactly. For cars, light vans and light trailers in Great Britain, current government guidance requires at least 1.6 millimetres across the central three-quarters of the tread and around the entire circumference. Other vehicle classes use different rules there, and other countries or states can define their own zones and thresholds. A recent inspection certificate is not a guarantee of current roadworthiness; GOV.UK says the driver remains responsible. Check the rule where the vehicle is operated rather than importing the standard from a video, rental-car home country or tire sidewall. [2][3]
Wet performance can deteriorate before a tire is legally worn out. As grooves become shallower, they have less capacity to channel water, increasing sensitivity to speed, standing-water depth, tire pressure, load and pavement texture. NHTSA rulemaking materials note that wet, snow and mud driving and braking forces decline as tread wears. No single depth predicts hydroplaning speed, and a new tire cannot cancel poor conditions. Slow substantially in rain, increase following distance, avoid abrupt steering or braking, and never use the legal minimum as a promise of new-tire grip. [6]
Read the wear pattern as diagnostic evidence. Heavy wear at both shoulders can accompany underinflation; center wear can accompany overinflation; one-sided or feathered wear can point to alignment or suspension problems; flat spots and cupping can signal other faults. NHTSA advises monthly checks of pressure, tread and damage and recommends professional attention for irregular wear, pressure loss, noise or vibration. Correcting the cause matters because installing another tire without repairing an alignment, load or pressure problem can reproduce the wear and compromise handling. [1][5]
Depth is only one retirement criterion. Stop and obtain professional inspection for exposed cord, a bulge, deep cut, cracking, puncture damage, repeated pressure loss, severe vibration or impact damage. Rubber also changes with service, storage, heat and sunlight; NHTSA notes that some vehicle and tire manufacturers recommend replacement after six to ten years regardless of tread and explains how to read the tire identification date code. Inspect the spare, maintain the vehicle maker’s cold inflation pressure for the load, and replace tires based on the worst relevant condition, not the best-looking measurement. [1][5]
Sources
- NHTSA — Tire Safety Ratings and Awareness
- GOV.UK — Check Your Vehicle Is Safe to Drive
- GOV.UK — MOT Inspection Manual: Tyres and Tread Depth
- NHTSA — FMVSS No. 139 Tread Wear Indicator Test Procedures
- NHTSA — Summer Driving and Road Trip Tips
- NHTSA — Consumer Tire Information Program Final Rule