U.S. · Trucking · Tires
DOT tire tread depth checker.
Steer tires on a bus, truck or truck tractor need at least 4/32 inch of tread. Every other tire needs 2/32. Enter your measurement.
Under 49 CFR 393.75(b), any tire on the front wheels of a bus, truck or truck tractor needs a tread groove pattern depth of at least 4/32 of an inch (3.18 mm); under 393.75(c) every other tire needs at least 2/32 of an inch (1.59 mm). Measure in a major tread groove, not where tie bars, humps or fillets sit.
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The two numbers in 393.75
49 CFR 393.75 is the federal tire rule for motor vehicles. Paragraph (b) says any tire on the front wheels of a bus, truck or truck tractor shall have a tread groove pattern depth of at least 4/32 of an inch when measured at any point on a major tread groove. Paragraph (c) says that, except as provided in (b), tires shall have a tread groove pattern depth of at least 2/32 of an inch when measured in a major tread groove. Both paragraphs say the measurement shall not be made where tie bars, humps or fillets are located.
| Position | Minimum depth | In millimeters | Paragraph |
|---|---|---|---|
| Front wheels of a bus, truck or truck tractor | 4/32 inch | 3.18 mm | 393.75(b) |
| All other tires | 2/32 inch | 1.59 mm | 393.75(c) |
The millimeter figures are converted from the inch figures (4/32 inch is 3.175 mm; 2/32 inch is 1.5875 mm), rounded to two decimals. The regulation states the limits in inches.
What counts as a "front wheel" tire
Paragraph (b) names the tires on the front wheels of a bus, truck or truck tractor. For a tractor-trailer that means the steer axle tires of the tractor. Drive axle tires on the tractor and every tire on the trailer fall under paragraph (c). The regulation does not use the word "steer", and the checker uses it only as shorthand for the front wheels described in (b). A truck with a single front axle has two tires subject to the 4/32 rule; each tire is measured on its own.
Defects that matter at any depth
Tread depth is only part of 393.75. Paragraph (a) bars operating a motor vehicle on a tire that has body ply or belt material exposed through the tread or sidewall, any tread or sidewall separation, that is flat or has an audible leak, or that has a cut to the extent that ply or belt material is exposed. A tire can pass the depth test and still fail on any of these.
Related tire rules in the same section
Front-wheel limits on retreads: no bus may be operated with regrooved, recapped or retreaded tires on the front wheels (393.75(d)), and a regrooved tire with a load-carrying capacity equal to or greater than 2,232 kg (4,920 pounds) may not be used on the front wheels of any truck or truck tractor (393.75(e)). Speed-restricted tires labeled with a maximum speed of 55 mph or less under FMVSS No. 119 may not be operated above that rated limit (393.75(f)). Tires may not carry a weight greater than the sidewall marking except under a State special permit at reduced speed, never above 80 km/hr (50 mph) (393.75(g)). No vehicle may be operated on a tire with a cold inflation pressure less than that specified for the load being carried (393.75(i)(1)); if the tire has been heated by recent operation, the cold pressure is estimated by subtracting the inflation buildup factor in Table 1 of the section (393.75(i)(2)).
Worked example
A driver measures 5/32 inch in the main groove of a steer tire and 3/32 inch on a drive tire. The steer tire meets the 4/32 minimum with 1/32 to spare, and the drive tire meets the 2/32 minimum with 1/32 to spare. After a few thousand miles the steer tire reads 3/32 inch. It is now 1/32 inch under the front-wheel minimum in 393.75(b) even though the same depth would still be legal on a drive axle. A reading of 4 mm converts to 5.04/32 inch, so it clears the front-wheel minimum with room to spare.
Limits
This page reads only 49 CFR 393.75 as current through October 29, 2026. It is not a roadside inspection guide: enforcement officers, state agencies and the Commercial Vehicle Safety Alliance use their own inspection and out-of-service criteria, which were not read for this page. Passenger car tires, trailers hauled by non-commercial vehicles and manufactured home loading are handled differently or by other paragraphs. The measurement you type is taken as accurate; use a proper tread depth gauge in the groove, not a coin.
Sources
Sources opened and read November 2, 2026.
Frequently asked questions
What is the minimum tread depth for a steer tire on a truck?
At least 4/32 of an inch (3.18 mm) of tread groove pattern depth, measured at any point on a major tread groove, on any tire on the front wheels of a bus, truck or truck tractor (49 CFR 393.75(b)).
What is the minimum tread depth for drive and trailer tires?
At least 2/32 of an inch (1.59 mm) of tread groove pattern depth in a major tread groove for all tires not covered by the front-wheel rule (49 CFR 393.75(c)).
Where on the tire do you measure tread depth?
In a major tread groove. The regulation says the measurement shall not be made where tie bars, humps or fillets are located (393.75(b) and (c)).
Can a bus run retreaded or regrooved front tires?
No bus may be operated with regrooved, recapped or retreaded tires on the front wheels (393.75(d)). A regrooved tire with a load-carrying capacity of 2,232 kg (4,920 pounds) or more may not be used on the front wheels of any truck or truck tractor (393.75(e)).
What other tire defects does 393.75 prohibit?
No motor vehicle may be operated on a tire with body ply or belt material exposed through the tread or sidewall, any tread or sidewall separation, a flat or audible leak, or a cut exposing ply or belt material (393.75(a)).
Does this page replace a roadside inspection standard?
No. It applies the 393.75 text. Inspectors also use their own out-of-service criteria, which this page does not read, so confirm with your carrier safety manager or the FMCSA.
Not legal or compliance advice
This page is general information drawn from the sources listed, not legal, tax or compliance advice, and it is not run by any government agency. Rules, forms and dates change and your agency, base jurisdiction or enforcement officer decides your case, so confirm with the office named on this page before you rely on it. Everything runs in your browser; nothing you enter is sent anywhere.