DOT trailer tire regulations live in 49 CFR 393.75, and they set enforceable minimums for tread depth, load capacity, cold inflation pressure, speed rating, and physical condition on every tire that turns under a commercial trailer.1eCFR. 49 CFR 393.75 – Tires The Federal Motor Carrier Safety Administration enforces the rule, and one tire that fails any of these standards can put a trailer out of service at the scale.
Conditions That Put a Trailer Out of Service Immediately
The most absolute part of 393.75 is paragraph (a). A trailer cannot operate on a tire that has body ply or belt material showing through the tread or sidewall, any separation of the tread or sidewall, a cut deep enough to expose cords or belts, or a flat tire or audible air leak.1eCFR. 49 CFR 393.75 – Tires There is no reduced-speed workaround. An inspector who finds one of these conditions writes the trailer out of service until the tire is changed.
Sidewall bulges, bumps, and knots fall in the same category as visible separation, because they signal internal structural damage from an impact. Tread separation tends to progress once it starts, and it often ends with rubber fragments in the traffic lane.
Minimum Tread Depth on Trailer Tires
Trailer tires must have at least 2/32 of an inch of tread in any major groove.1eCFR. 49 CFR 393.75 – Tires The measurement can’t be taken on a tie bar, hump, or fillet, because those molded features sit higher than the surrounding groove and produce an inflated reading. A tread depth gauge in the deepest part of the groove is the accepted method.
Steering axle tires on trucks and buses face a stricter 4/32-inch floor under paragraph (b), so the 2/32 number is specific to non-steer positions.1eCFR. 49 CFR 393.75 – Tires Two thirty-seconds is the bare legal minimum. A tire sitting at that depth has already lost most of its water-channeling capacity, which is why fleet practice usually pulls trailer tires well before they reach the line.
Load Rating and Overload Rules
Under 49 CFR 393.75(g), a trailer cannot operate on tires carrying more weight than the load rating stamped on the sidewall.1eCFR. 49 CFR 393.75 – Tires If the sidewall lacks a weight marking, the limit defaults to the load rating published by one of the tire industry organizations referenced in FMVSS No. 119. Overloaded tires build excessive heat, and heat is what breaks a carcass down internally.
One narrow exception exists. A state can issue a special permit authorizing operation above the rated load, but only if the trailer travels at a reduced speed that never exceeds 50 mph.2eCFR. 49 CFR 393.75 – Tires Outside that permit, running an overloaded tire is a straight violation. Check axle weight against the combined load rating of the tires on that axle, because a load that shifts to one side can push a single tire past its rating even when the total looks fine.
Cold Inflation Pressure
Paragraph (i) prohibits operating a trailer tire whose cold inflation pressure sits below the level specified for the load it carries.1eCFR. 49 CFR 393.75 – Tires “Cold” means the tire hasn’t been heated by recent driving. Because highway running raises internal pressure, the regulation includes an inflation buildup factor that inspectors subtract from a measured pressure to estimate what the tire would read cold.
That subtraction catches drivers off guard at roadside checks. A tire reading 100 psi after a hundred miles of running may compute out to something well under the required cold minimum once the buildup factor comes off. Set pressures before the first trip of the day, when the tires haven’t moved. A tire measured at less than 50 percent of its maximum rated pressure is treated as flat under CVSA criteria and triggers an immediate out-of-service order.
Speed-Restricted Tires
Some commercial tires carry a label capping their maximum speed at 55 mph or less. Under 49 CFR 393.75(f), a trailer wearing one of these tires cannot be driven faster than the labeled speed.1eCFR. 49 CFR 393.75 – Tires It’s an equipment violation on its own, separate from any visible damage. A 55-mph tire on a trailer running 65 heats up beyond what the carcass was built to handle.
Retreads, Recaps, and Regrooved Tires
Retreaded, recapped, and regrooved tires are permitted on trailer positions. The restrictions in 393.75 apply to front wheels: buses cannot run regrooved, recapped, or retreaded tires on front wheels under paragraph (d), and trucks or truck tractors cannot use regrooved tires rated at 4,920 pounds or more on front wheels under paragraph (e).1eCFR. 49 CFR 393.75 – Tires Neither paragraph limits these tires on trailer axles.
Regrooving is the process to know. It cuts new tread into a worn casing, and it’s only safe on tires that were manufactured with extra rubber below the original tread pattern. A “regroovable” marking on the sidewall means the tire was built for that process. Regrooving a tire that wasn’t designed for it cuts into the structural cords. Whichever category a trailer tire falls into, it still has to meet the 2/32-inch tread minimum and pass every condition in 393.75(a).
Sidewall Markings and Tire Age
FMVSS No. 119, at 49 CFR 571.119, sets the marking and performance requirements for tires on vehicles rated above 10,000 pounds GVWR, which covers most commercial trailers.3eCFR. 49 CFR 571.119 – Standard No. 119 Compliant tires show a DOT symbol on the sidewall, a Tire Identification Number that encodes the plant, size, and production date, and the maximum load rating and inflation pressure.
The last four digits of the TIN give the week and year of manufacture. Trailer tires often see low mileage but sit outdoors for years, and rubber degrades with age regardless of tread. No federal regulation sets a hard age limit, but industry practice treats trailer tires older than seven to ten years as replacement candidates even when tread depth still passes.
Roadside Inspections, Fines, and CSA Consequences
Roadside enforcement runs on Commercial Vehicle Safety Alliance procedures. A Level I inspection is the full check of vehicle and driver, including every tire on every axle.4Commercial Vehicle Safety Alliance. All Inspection Levels Any violation that meets CVSA out-of-service criteria stops the trailer where it sits until the defect is fixed or the tire is replaced.
The violations that most often trigger an out-of-service order are exposed belt material, tread or sidewall separation, flat tires, and tread below 2/32. Fines generally run from $100 to $500 per tire depending on the state. The violation also feeds into the FMCSA’s Compliance, Safety, Accountability system, where points in the Vehicle Maintenance category raise audit risk and eventually insurance costs.
Pre-Trip Inspection and Driver Responsibilities
Most tire violations that get caught at a scale were visible in the yard. Federal rules require commercial drivers to conduct a pre-trip inspection covering every tire on the vehicle and trailer, checking for visible damage, tread depth, and inflation. A tire that looks soft, wears unevenly, or shows any sidewall damage needs attention before the trailer rolls.
Dual assemblies are where problems hide. When two tires share a position, a pressure mismatch makes the higher tire drag the lower one, wearing both. The inner tire is partially blocked by the outer, so slow leaks and low pressure go unnoticed the longest. A long-reach gauge or a hand along the inner sidewall at each stop catches most of it.
Carriers that keep tire inspection records per trailer, with pressures, tread measurements, and replacement dates, have a much stronger position in an audit or after an incident. That documentation is often the line between a routine investigation and a negligence claim.