Prevention FAQ — FMCSR 393.75B: Steer Axle Tire Tread Depth

Inspectors are measuring tread depth on the steer axle tires — the threshold is 4/32 inch. They use a tread depth gauge and typically probe the major grooves at multiple points across the tire width, not just the center. They will also check for uneven wear patterns that concentrate tread loss on one shoulder, which can put a tire below threshold even when the center still appears acceptable.

Our inspection records show Texas is by far the highest-enforcement state, generating 719 citations in the last 180 days alone — more than 23 times the volume of the next closest state, Iowa (31 citations). If your fleet runs cross-border or Trans-Texas corridors, inspectors there have clearly made steer tire condition a priority. Expect inspectors at weigh stations and Level I screenings to pull a gauge if any visible cupping, uneven wear, or sidewall cracking is present on the steer axle.

Add the following as distinct, checkable line items — not a generic 'tires OK' checkbox:

1. Tread depth gauge reading on both steer tires — record the actual measurement (in 32nds) in at least two tread grooves per tire, including the edge grooves.
2. Wear pattern flag — note any cupping, center wear, or one-sided shoulder wear and escalate immediately.
3. Minimum action threshold — set an internal pull-point at 5/32 inch so tires never reach the legal 4/32 line before the next inspection cycle.
4. Visual check for sidewall damage, bulging, or embedded objects that accelerate tread loss.

FREQHT trucks account for 755 all-time citations under this code — more than any other vehicle make in our database. If your fleet runs Freightliners, make sure your pre-trip training specifically references steer tire depth measurement as a required step, not an optional visual glance.

Drivers should carry:

• The most recent completed DVIR showing steer tire tread depth measurements, dated and signed.
• Any repair or tire replacement work orders completed within the last 30 days, especially if tires were recently replaced.

Carriers should retain in the vehicle file:

• Tire installation records showing the make, size, and date of mounting for each steer axle tire.
• Tread depth logs from pre-trip inspections going back at least 90 days — this creates a wear-rate record that can support a DataQs challenge if a measurement is disputed.
• Periodic inspection certificates (FMCSR 396.17(c)) — our co-occurrence data shows 48 shared inspections in the last 90 days where 396.17C (no proof of periodic inspection) was cited alongside 393.75B, meaning carriers are compounding their exposure by failing to retain basic paperwork.

Our inspection records reveal three systemic patterns in the last 90 days:

1. 393.9 (Inoperable Required Lamp) — 118 shared inspections. The most common co-occurring code by a wide margin. When lights and steer tires fail together, it points to a pre-trip inspection process that is not being completed at all — or completed superficially. Drivers are leaving the yard with multiple unchecked defects simultaneously.

2. 393.75C (Tread depth insufficient on other axles) — 65 shared inspections. When steer and non-steer axle tires are both below threshold on the same vehicle, the root cause is a fleet-level tire replacement program that is reactive (fix it when it fails) rather than proactive (replace on wear rate or mileage schedule).

3. 392.2RG (Operating while ill or fatigued) — 64 shared inspections. This pairing suggests dispatch pressure or driver fatigue culture that also manifests as skipped or rushed pre-trips. A fatigued driver is unlikely to crouch down with a tread gauge before a run.

Do not rely solely on the shop work order. Require the following before releasing the vehicle:

1. Post-repair tread depth measurement on both steer tires, recorded by a shop technician or your fleet manager — not the driver who was cited.
2. Photograph of the gauge reading in each major groove, saved to the vehicle file with date and odometer.
3. Confirmation that the replacement tire meets or exceeds the original equipment specification for load range and size — downgrades are a compliance risk on the steer axle.
4. Update the DVIR to reflect the repair and the confirming measurement.

Because this code carries a CSA severity weight of 7, a single uncorrected citation can move your Vehicle Maintenance BASIC meaningfully. Getting the return-to-service documentation right also positions you to challenge borderline citations through DataQs if the recorded measurement is ever disputed.

Run the following within 72 hours of the citation:

1. Pull the cited vehicle immediately and independently measure both steer tire tread depths. Document the findings.
2. Review the driver's last 30 days of DVIRs — was the tire ever flagged? If not, the inspection process failed; if yes, was the defect corrected?
3. Check every other vehicle the cited driver operates — if one was missed, others may be marginal.
4. Identify the wear rate — at what mileage interval did this tire drop below threshold? Compare to your replacement schedule.
5. Review the co-occurrence context — our data shows 393.9 appeared in 118 of the same inspections in the last 90 days. If your cited inspection also included a lamp violation or any other defect, that is a signal of a systemic pre-trip failure, not an isolated tire oversight.
6. Document corrective action in the driver's qualification file and in the vehicle maintenance file.

This code carries a CSA severity weight of 7, which is on the higher end of the Vehicle Maintenance BASIC scale. Points are time-weighted — a citation in the last 6 months carries full weight; severity erodes over 24 months.

With 2,959 all-time citations in our database, 393.75B ranks #442 out of 3,036 FMCSR codes by volume — it is not an obscure code, and FMCSA's SMS algorithm treats it accordingly. For context, peer Vehicle Maintenance codes like 396.3(a)(1) (general inspection/repair/maintenance) carry a 45.3% OOS rate; 393.75B's 16.0% OOS rate is below the all-FMCSR average of 31.4%, meaning inspectors frequently cite without placing the vehicle out of service — but the severity weight still posts to your BASIC. A fleet accumulating multiple 393.75B citations will see compounding score movement even without OOS events.

Training should cover three practical skills:

1. How to use a tread depth gauge correctly — angle, placement in the groove, reading in 32nds, and how to interpret an edge groove reading versus a center groove reading.
2. Internal action threshold vs. legal minimum — drivers must understand that 4/32 inch is the citation threshold, and that the fleet's pull-point is 5/32 inch (or whatever your internal standard is). Waiting until they see the legal line means the margin for a repair delay has already been consumed.
3. Wear pattern recognition — cupping from alignment issues, one-sided wear from inflation imbalance, and center wear from chronic overinflation all create tread depth asymmetry that a simple center-groove probe misses.

Prioritize this training on FRHT (755 citations), KW (385 citations), and INTL (236 citations) operators in your fleet — those three vehicle makes account for the largest citation volumes in our database and should be the first cohort through any refresher.

File a DataQs challenge when you have documented evidence that contradicts the cited measurement. Specifically:

• You have a pre-trip DVIR from the same day showing a tread depth measurement at or above 4/32 inch, with the gauge reading recorded numerically.
• A post-inspection measurement taken within hours of the citation shows a reading at or above the threshold — photograph with timestamp.
• The inspector recorded the wrong axle — the citation specifies the steer axle; if the deficient tire was on a drive or trailer axle, the code is incorrect.
• Tire replacement records show the steer tires were replaced after the cited pre-trip but before the inspection, and the shop paperwork confirms compliant tread depth at installation.

Do not file a challenge if you cannot produce documentation — a challenge without supporting records will be rejected and wastes administrative time. Build the documentation habit described in the paperwork FAQ above so that the evidence exists before you need it.

Our inspection records for the last 12 months show a persistently elevated citation rate: the lowest single-month total (excluding the partial month of April 2025 with 47 citations) was 118 in January 2026, and the highest was 191 in October 2025. The last 90 days generated 380 citations — nearly 21% of the 1,796 recorded in the full 12-month period — which means this is not a seasonal spike; it is a year-round, sustained enforcement priority.

Recommended audit cadence:

• Every pre-trip: Driver-level gauge measurement with recorded depth, both steer tires.
• Every 15,000 miles or 30 days (whichever comes first): Maintenance-level audit with depth logged to the vehicle file.
• Quarterly fleet-wide audit: Pull tread depth records for all vehicles and flag any approaching the 5/32-inch internal threshold.

Given that the 90-day citation count of 380 is tracking at a rate consistent with — or slightly above — the 12-month pace of 1,796, do not lengthen your audit intervals.