The short version. Under 49 CFR 393.75, any tire on the front (steer) wheels of a truck, truck tractor or bus needs at least 4/32 inch of tread, and every other tire needs at least 2/32 inch. Measure in a major tread groove, never on a tie bar, hump or fillet. Those are the legal floor, not a replacement plan.

The federal minimums

Tire positionMinimum tread depthRule
Front (steer) wheels of a truck, truck tractor or bus4/32 inch, at any point on a major tread groove393.75(b)
Every other tire: drive axles, trailers, converter dollies2/32 inch, measured in a major tread groove393.75(c)

Roadside inspectors also work from the Commercial Vehicle Safety Alliance’s out-of-service criteria, which decide when a vehicle is taken off the road on the spot. A tire can be a violation of 393.75 without the truck being put out of service, and the violation still goes on the inspection report.

Tire conditions that are illegal at any depth

Plenty of tread does not make a tire legal. Section 393.75 also prohibits running 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 ply or belt material.

And a few rules depend on where the tire is and how it is used:

  • Buses: no regrooved, recapped or retreaded tires on the front wheels.
  • Trucks and truck tractors: no regrooved tire with a load-carrying capacity of 4,920 pounds or more on the front wheels.
  • Speed-restricted tires labeled for 55 mph or less may not be run faster than their rated limit.
  • Load: no tire may carry more than the weight marked on its sidewall, unless the vehicle runs under a state special permit at reduced speed — and never above 50 mph.
  • Inflation: cold inflation pressure may not be below what is specified for the load being carried. A reading taken hot is corrected using the table in the regulation.

How to measure tread depth

Use a tread depth gauge that reads in 32nds of an inch. Push the probe into a major tread groove — one of the main channels running around the tire — and read it with the gauge’s shoulders flat on the tread.

  • Stay off tie bars, humps and fillets. These raised sections at the bottom of some grooves sit above the groove floor, so a reading taken on one does not reflect the real depth. The regulation excludes them.
  • Measure across the tread. Take the inside, center and outside grooves, at a few points around the tire.
  • Go by the lowest reading. For steer tires the rule applies at any point on a major groove, so one low spot is enough to fail.
  • Read the wear pattern. One shoulder lower than the other, or a tire wearing faster than its partner on the same axle, usually points to alignment or inflation. Fix the cause, not just the tire.

Penny and quarter checks

When there is no gauge in the truck, coins give a rough answer. Put a penny into a groove with Lincoln’s head pointing down: if you can see all of his head, the tread is at or below about 2/32 inch (Tire Rack). A quarter does the same job for steer tires: if the top of Washington’s head shows, the tread is at or below about 4/32 inch (Brake & Front End). Coins are a spot check; a gauge reading is what belongs on the record.

The legal minimum is not your replacement line

A steer tire that measures 5/32 inch today can be under 4/32 before anyone looks at it again. Set two lines of your own for each position:

  • A warn line, where the tire goes on the watch list and a replacement gets scheduled.
  • A replace line, where it comes off — high enough that it cannot wear past the legal minimum in the miles a vehicle covers between inspections.

A fleet that inspects weekly can run closer to the minimum than one that checks tires once a month. The right lines come from your own wear rates, which is the best argument for recording real readings.

Vans and light trucks

Part 393 covers commercial motor vehicles — broadly, vehicles of 10,001 pounds or more in interstate commerce (49 CFR 390.5) — and many states apply the same standards to intrastate carriers. Cargo vans, pickups and other light vehicles below that line fall under state law, where 2/32 inch is the most common minimum. Leased vehicles can face a stricter standard at turn-in: GM Financial’s published wear guide, for example, counts tread under 4/32 inch as excess wear. See the lease-return inspection checklist.

Tracking tread across a fleet

Record four things every time a tire is measured: the reading at each wheel position, the date, the odometer and who took it. After a few inspections that gives you a wear rate per position, which turns “the tires look fine” into “the right front hits our replace line in about three weeks.”

  • Watch for one position wearing faster than its partner on the axle.
  • Compare wear between vehicle types and routes — duty cycle shows up in tires first.
  • Notice the vehicle that is back on the list after every inspection.

Tread depth, wheel by wheelMotentra records tread depth in 32nds of an inch at inspection, flags front left, front right, rear left and rear right individually, and judges every reading against the warn and replace lines you set. Anything under your lines shows on the fleet list.

See vehicle inspections

Frequently asked questions

What is the minimum tread depth for steer tires on a truck?
4/32 inch, measured at any point on a major tread groove, for 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?
2/32 inch, measured in a major tread groove and away from tie bars, humps and fillets (49 CFR 393.75(c)).
Where should tread depth be measured?
In a major tread groove, never on a tie bar, hump or fillet. Measure the inside, center and outside grooves at a few points around the tire and go by the lowest reading.
Can a truck run retreads on the steer axle?
Section 393.75 bars regrooved, recapped and retreaded tires from the front wheels of buses, and regrooved tires with a load capacity of 4,920 pounds or more from the front wheels of trucks and truck tractors. It does not prohibit retreaded tires on a truck’s front wheels. Check any state rules that apply to you.
Is the penny test accurate enough?
As a spot check. If you can see all of Lincoln’s head, the tread is at or below about 2/32 inch; for steer tires, the top of Washington’s head on a quarter marks about 4/32 inch. For records, use a tread depth gauge.

Sources. eCFR: 49 CFR 393.75 and 390.5; Tire Rack on the penny test; Brake & Front End on the quarter test; GM Financial’s lease wear and use guidelines. This guide summarizes federal rules for general information; it is not legal advice. Check the current regulations and any state rules that apply to you.

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