Walk up to a trailer tire and it usually looks fine. The lugs are there, the rubber is black, and no cord is showing. Then an inspector puts a gauge into one groove on the shoulder side and reads a number that ends the conversation. That situation is what 49 CFR 393.75(c) is written for.
The rule, as it appears in the violation data I work from, reads: "Tire-other tread depth less than 2/32 of inch measured in a major tread groove." The code is 393.75(c), it sits in the Vehicle Maintenance BASIC, and it belongs to the Tires violation group. Its severity weight is 8. It is not flagged as an out-of-service indicator on its own. The full record is on the code 1034 page.
What the threshold is measuring
Tread depth is read in 32nds of an inch. The 2/32 threshold applies to "other" tires, meaning anything that is not a steer-axle tire. Steer tires fall under a separate, stricter threshold in the same section. Do not rely on a shop rule of thumb for the front axle. Pull the current text of 393.75 and use the figure written there.
The rule specifies a major tread groove for a reason. Major grooves are the deepest channels in the tread pattern, and they are the channels that carry water away from the contact patch. Readings taken on a sipe, a shallow secondary cut, or the edge of a lug can look acceptable while the channel that matters is nearly gone. The regulation tells you where to measure so the number reflects the part of the tire that is supposed to keep it on the road in the wet.
Where the gauge actually goes
The method is simple. Choose a major groove, seat the gauge to the bottom of the channel, and read it. The difficulty is coverage. Uneven wear is common, and it is usually a symptom of something upstream: misalignment, low or uneven inflation, a dragging brake, or a worn steer component. A tire can have a healthy center rib and a dangerously worn outer groove. One good reading does not clear the tire if another major groove on the same tread is down.
Many tires also carry wear indicator bars molded into the groove bottoms. When the tread surface sits level with those bars, the tire has reached the wear point. Treat the bars as a quick visual check, not a substitute for a measurement. If you are in doubt, measure.
Why this shows up so often
In the data I have, this code shows 230,221 recorded violations. Of those, 7,845 were out-of-service, which works out to an OOS rate of 3.4 percent. The OOS flag on the code is false, so the code is not an automatic out-of-service violation. The out-of-service share is what came out of real inspections, and it tells you that most tread write-ups stay as write-ups, but a meaningful minority did not.
The volume matters more than the OOS share for most carriers. A tread finding is cheap to avoid and easy to spot on a roadside walkaround, which is why it is one of the most frequent findings in the Tires group.
What it does to your SMS score
Vehicle Maintenance is a BASIC used in SMS, and this code's severity weight of 8 feeds directly into it. The calculator applies time weights that decline as the violation ages. Based on the constants I'm using:
- 0 to 6 months: time weight 3, so 8 × 3 = 24
- 6 to 12 months: time weight 2, so 8 × 2 = 16
- 12 to 24 months: time weight 1, so 8 × 1 = 8
- Beyond 24 months: time weight 0
The calculator also caps severity at 30 per inspection per BASIC, so one bad inspection cannot add unlimited weight. A single tread write-up is well under that cap, but the cap matters on inspections with several findings in the same BASIC. The point to take from the table is timing: the violation hurts most in the first six months and fades completely after two years.
What to do before the next inspection
Measure every tire yourself, not just the ones that look worn. Check at least one major groove across the full width of each tread, including inner duals, and write the readings down. If a tire is close to the threshold, look at the cause before you replace it: inflation records, alignment, and brake adjustment will tell you whether the next set will wear the same way.
If you want to see how a single tread write-up moves a carrier's BASIC over time, the calculator uses these same constants. For terms like BASIC and time weight, the glossary covers them in plain language.