The rule
49 CFR 393.75 sets tire requirements for commercial motor vehicles, including tread depth. Appendix A to Part 396, the minimum periodic inspection standards used for the DOT annual inspection, repeats the limits as reject criteria:
- Steering axle of a power unit: a tire fails with less than 4/32 inch of tread when measured at any point on a major tread groove.
- All other tires: the limit is 2/32 inch.
Other steering tire reject conditions
Appendix A also rejects a steering axle tire on a power unit that has body ply or belt material exposed through the tread or sidewall, any tread or sidewall separation, a cut exposing ply or belt material, a "Not for Highway Use" marking, mixed bias and radial tires on the same axle, a regrooved tire (except in limited urban or suburban service), a boot, blowout patch or other ply repair, weight above the tire load limit (including from low air pressure), a flat or audible or felt leak, or contact with any part of the vehicle. Buses may not run recapped or retreaded steering tires.
How to measure
Use a tread depth gauge in a major groove, not on a wear bar or in a shallow sipe, and check several points around the tire, because the rule fails the tire at any point below the limit. Record the reading per position on the inspection or work order.
For repair shops
Tread depth is one of the most common findings on annual inspections and PMs. Writing the measured depth by tire position, rather than "tires OK," gives the fleet a defensible record and gives you a reason to quote replacements before a failure. See the DOT annual inspection checklist. MetaFleet runs digital inspections from your own form; see inspections and DVIRs.