Tire Tread Depth Rules for Trucks (49 CFR 393.75)

What 49 CFR 393.75 requires

Each line below is from the regulation's own text in 49 CFR 393.75. The paragraph letter tells you where to read the exact words.

Rule	Applies to	Paragraph
No tire with body ply or belt material exposed through the tread or sidewall, any tread or sidewall separation, a flat or an audible leak, or a cut deep enough to expose ply or belt material	Every motor vehicle	(a)
Tread groove depth of at least 4/32 inch, measured at any point on a major tread groove	Tires on the front wheels of a bus, truck or truck tractor	(b)
Tread groove depth of at least 2/32 inch, measured in a major tread groove	All other tires	(c)
No regrooved, recapped or retreaded tires on the front wheels	Buses	(d)
No regrooved tire with a load-carrying capacity of 4,920 pounds (2,232 kg) or more on the front wheels	Trucks and truck tractors	(e)
Speed-restricted tires labeled with a maximum speed of 55 mph or less may not be run faster than their rated limit	Every motor vehicle	(f)
No tire may carry more weight than its sidewall marking allows, unless the vehicle runs under a state special permit at reduced speed, and then never above 50 mph	Every motor vehicle except manufactured homes, which have their own limits	(g), (h)
Cold inflation pressure may not be lower than the pressure specified for the load being carried	Every motor vehicle	(i)

Two points are easy to misread. First, the front-wheel retread ban in paragraph (d) applies to buses only; for trucks and truck tractors, paragraph (e) restricts regrooved tires above the 4,920-pound rating on the front wheels. Second, the steer-tire wording is stricter than the other: 4/32 inch "at any point on a major tread groove", against 2/32 inch "in a major tread groove" for the rest.

Measuring tread depth the way the rule reads

Use a gauge that reads in 32nds of an inch. The rule is written in 32nds, and a reading in millimeters has to be converted before it can be compared.
Measure in a major tread groove. Both paragraphs say the measurement may not be taken where tie bars, humps or fillets are located, so set the probe in the open groove beside them.
On front tires, look for the lowest point. Because the 4/32 rule applies at any point on a major groove, one worn patch can fail a tire whose average depth is fine. Check several places around the circumference, and both edges as well as the center.
Write the reading down against the wheel position. A depth with no position and no date cannot show a trend, and cannot show that a tire was legal when it left the yard.

Checking pressure when the tire is hot

The pressure rule is about cold inflation pressure. A tire that has just been driven reads higher, so paragraph (i) gives a correction: estimate the cold pressure by subtracting the buildup in the regulation's Table 1. After an hour at an average of 41 to 55 mph, that buildup is 5 psi for tires with a maximum load rating of 4,000 pounds or less, and 15 psi for tires rated above 4,000 pounds. A gauge reading taken at a fuel stop is therefore not the number to compare with the load chart until it has been corrected.

Who is responsible for checking

The driver, before every trip. 49 CFR 392.7(a) lists tires, and wheels and rims, among the parts a driver must be satisfied are in good working order before driving, and 49 CFR 396.13 requires the driver to be satisfied the vehicle is in safe operating condition. A defect found then belongs on the driver vehicle inspection report.
The motor carrier, all the time. 49 CFR 396.3(a) requires every carrier to systematically inspect, repair and maintain the vehicles it controls, and to keep parts and accessories in safe and proper operating condition at all times. Tire checks belong in the preventive maintenance schedule, and the results in the vehicle maintenance file.

A pre-trip tire check

Check	What fails	Rule
Tread on each front tire	Below 4/32 inch at any point on a major groove	393.75(b)
Tread on drive and trailer tires	Below 2/32 inch in a major groove	393.75(c)
Sidewalls and tread face	Exposed ply or belt, separation, bulges, a cut down to the cords	393.75(a)
Listen and look for leaks	A flat or an audible leak	393.75(a)
Pressure, cold if possible	Below the pressure specified for the load	393.75(i)
Front tires on a bus	Any regrooved, recapped or retreaded tire	393.75(d)
Front tires on a truck or tractor	A regrooved tire rated at 4,920 pounds or more	393.75(e)
Load against the sidewall rating	More weight than the marking allows	393.75(g)
Duals, inner tire included	Any of the conditions above on the inner dual, which is the hardest tire to see	393.75(a)–(c)

Keeping a tire history that is worth having

A tire record is most useful when each tire is tracked as an object, not as "the left front": its serial number, its DOT date code (which gives its age), the wheel position it sits on, every tread and pressure reading, and the date and reason it came off. That history shows which positions wear fastest, which brands last, and whether a tire that failed an inspection had been reading low for weeks. Cargovate keeps that kind of record: each tire at its wheel position with its serial number and DOT date code, tread readings checked against 49 CFR 393.75, and the tires that break a rule listed for the whole fleet in one place.

A tire that breaks these rules can also be found by an inspector at the roadside. See roadside inspections and out-of-service orders for what happens next, and the glossary for the terms used here.