Truck tire tread depth: legal minimums in the US and Canada

US rules: what 49 CFR 393.75 says

In the US, tire condition and tread depth for commercial motor vehicles are set by 49 CFR 393.75, part of the Federal Motor Carrier Safety Regulations. The section has two kinds of rules: conditions that ban a tire outright, and minimum tread depths by position.

No motor vehicle may be operated on a tire that:

• has body ply or belt material exposed through the tread or sidewall;
• has any tread or sidewall separation;
• is flat or has an audible leak; or
• has a cut deep enough to expose the ply or belt material.

The tread depth minimums depend on where the tire sits:

• Front wheels of a bus, truck or truck tractor: at least 4/32 inch, measured at any point on a major tread groove.
• All other tires: at least 2/32 inch, measured in a major tread groove.
• Neither measurement may be taken where tie bars, humps or fillets are located.

The same section also covers load and speed. A tire may not carry more weight than the load marked on its sidewall, except under a state special permit at reduced speed, and never above 50 mph in that case. Speed-restricted tires labelled for 55 mph or less may not be run faster than their limit. And no tire may be operated at a cold inflation pressure below the pressure specified for the load it carries.

Tires are also on the list every US driver vehicle inspection report has to cover under 49 CFR 396.11, alongside brakes, steering, lights, wheels and rims.

The out-of-service line at the roadside

The regulation is one line; the out-of-service criteria that roadside inspectors apply are another. The Commercial Vehicle Safety Alliance (CVSA) publishes the North American Standard Out-of-Service Criteria. A CVSA inspection bulletin on measuring tread depth quotes the tire criterion for steering axles: a tire on a front steering axle of a power unit is out of service when it has less than 2/32 inch (1.6 mm) of tread in any two adjacent major tread grooves, typically the grooves that contain a tread wear indicator, at any location on the tire. Measurements are not taken on stone ejectors or tread wear indicators.

That leaves a gap. A steer tire at 3/32 inch will not park the truck, but it is below the 4/32 inch minimum in 393.75 and will be written up as a violation. Plan replacements around the regulation, not the out-of-service line. The criteria are updated every year, so check the current edition for the full tire section.

Tires are one of the most common reasons trucks are placed out of service. They were the vehicle focus of CVSA’s 2025 International Roadcheck, and inspectors recorded 2,899 tire-related out-of-service violations, 21.4% of all vehicle out-of-service violations and second only to brake systems. In the US alone the share was 23.4%; in Canada it was 9.5%. CVSA lists flat tires, insufficient tread depth, severe cuts exposing cord ply or other damage causing bulges, improper repairs, and tires not suited to the load or to highway use among the violations it finds.

Canadian rules: NSC Standard 11, Schedule 1 and the provinces

In Canada, the National Safety Code (NSC) sets the national standards, and every province and territory adopts them by reference or mirrors them in its own law. Two standards cover tires.

NSC Standard 11: maintenance and periodic inspection

Part A of Standard 11, the maintenance standard, sets these limits:

• No front tire with tread depth less than 3 mm, and no rear tire with tread depth less than 2 mm. Vehicles carrying dangerous goods may need a different minimum under provincial rules.
• No retreaded tire on an active steering axle, and no regrooving of a tire not marked “Regroovable”.
• No cut or crack longer than 25 mm that goes deeper than a major tread groove or into the casing ply, and no exposed body cord.
• No radial tire mixed with a non-radial on the same axle, and no tire labelled “Not for Highway Use” on a public road.
• No tire 10% above or below the recommended inflation pressure, and no pressure difference of more than 10% between dual-mounted tires.

Part B, the periodic motor vehicle inspection standard, rejects a vehicle when a tire on an active steering axle has less than 3 mm of tread, or any other tire has less than 2 mm. It lists less than 2 mm on a front tire and less than 1 mm on a rear tire as hazardous conditions, and notes that some jurisdictions require at least 3 mm on all tires of certain vehicles.

NSC Standard 13: the daily trip inspection

Schedule 1 of Standard 13, the daily trip inspection for trucks, tractors and trailers, lists a damaged tread or sidewall and a leaking tire as defects; a leak that can be felt or heard is treated as a flat. It lists as major defects, which mean the vehicle may not be driven: a flat tire, tread depth below the wear limit, a tire touching another tire or any part of the vehicle other than the mud flap, a tire marked “Not for highway use”, and exposed cords in the tread or outer sidewall.

Ontario

Ontario’s Ministry of Transportation truck handbook states that the rear tires of a motor vehicle must have at least 1.5 mm of tread and the front tires of a motor vehicle with a manufacturer’s gross vehicle weight limit above 4,500 kg must have at least 3 mm, both measured in two adjacent tread grooves. Ontario’s rear figure differs from the NSC’s 2 mm, which shows why you should check the rules of each province your trucks are plated in or run through.

How and where to measure tread depth

The US, CVSA and NSC rules all measure in a major tread groove: one of the deepest moulded grooves around the tire, the kind that contains the wear bars. The NSC periodic inspection standard tells the inspector to find the area where the tread is lowest, measure there with a tread depth gauge, and record the lowest reading. CVSA’s bulletin on evolving tread designs describes the gauge routine step by step:

1. Check that the gauge works and that the probe is narrow enough to fit in the main grooves.
2. Push the gauge against a hard, flat surface and confirm it reads zero when fully compressed.
3. Depress the plunger fully.
4. Place the probe in the centre of a circumferential groove and push down on the gauge’s base.
5. Do not measure on tread wear indicators or other raised parts of the tread design. Under 393.75, do not measure on tie bars, humps or fillets either.
6. Remove the gauge by its barrel without touching the probe.
7. Read the right scale: most gauges show both millimetres and 32nds of an inch.
8. Repeat across the other main grooves of the tire.

Take readings at more than one point around the tire as well as across it. The US front-tire rule applies at any point on a major groove, so one low spot from irregular wear is enough to fail.

Some newer tires have hidden grooves that open up as the tread wears. CVSA notes that these grooves are full depth from the first millimetre to the last, can be checked with an ordinary gauge, and that North America currently has no minimum groove width, only a minimum depth.

A tread log you can actually use

A tread log turns single readings into a trend. With the same gauge, the same units and the same positions every time, you can see which tires are wearing fast, spot alignment or inflation problems early, and order tires before a truck is stuck waiting for them.

Set the rules for the log once and write them down:

• When to read: at every PM service, at every tire mount or removal, and after any roadside tire violation. Drivers still check tires on every trip inspection.
• What to record: unit, position, date, odometer and the lowest reading for each tire, plus the tire’s serial number or brand and size so a tire can be followed when it moves.
• Your pull point: a company replacement point above the legal minimum, for example a higher figure for steer tires, so a tire never reaches the legal line between two readings.
• Removal reason: worn out, damaged, irregular wear or sent for retread. Over time this shows you where tires are being lost.

Retreads, regrooving and where they may run

Retreading extends the life of a casing, but the steer axle is where the US and Canadian rules part ways.

• US: 49 CFR 393.75 bars regrooved, recapped or retreaded tires on the front wheels of a bus. For trucks and truck tractors, it bars regrooved tires with a load-carrying capacity of 2,232 kg (4,920 pounds) or more on the front wheels. It does not bar retreaded front tires on trucks.
• Canada: NSC Standard 11 bars a retreaded tire on any active steering axle, and rejects retread material that is loose, missing or separated from the casing. Regrooving is allowed only on tires marked “Regroovable”.

So a retreaded steer tire that is legal on a US-only truck fails the NSC periodic inspection criteria. A fleet that crosses the border, or might sell trucks into Canada, is simpler to run with new tires on every steering axle and retreads only on drive and trailer positions.

Tire cost per mile

Cost per mile is the fairest way to compare tires, retreaders and positions, because a cheap tire that wears out early can cost more than an expensive one that runs longer. For each tire:

1. Add up what the tire cost you: the purchase, mounting and balancing, repairs, and each retread.
2. Take the miles (or kilometres) it ran from mount to final removal, from the odometer readings at each mount and removal.
3. Divide the cost by the distance.

Track it by position and by tire model, because steer, drive and trailer tires wear differently. Compare retread vendors on cost per mile of the retread life alone, and watch for tires that come off early for damage: they can point to a route, inflation or alignment problem rather than a tire problem.