10.5 Tire Specification Inspection: Sidewall Markings, DOT Date Codes, Tread Depth & Load/Speed Ratings

10.5 Tire Specification Inspection: Sidewall Markings, DOT Date Codes, Tread Depth & Load/Speed Ratings

Task D5 on the official ASE T5 task list is one of the longest single tasks in the guide: inspect and measure tire condition, date codes, tread depth, tire diameter and circumference, and valve stems and caps; match tires and rims; adjust air pressure; verify proper application, speed rating, and load range. Diameter, circumference and inflation are covered in Section 10.3. This section covers the rest — the sidewall, the legal minimums, and the matching rules — and almost every fact here is a federal regulation, which makes it prime exam material.

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Reading a Commercial Tire Sidewall

Size Designations

Two formats dominate commercial service, and mixing them up on a spec sheet is a real-world error:

The aspect ratio is what makes a "low profile" tire low: a 295/75R22.5 has a shorter sidewall and a smaller overall diameter than an 11R22.5 even though both fit a 22.5-inch rim. That diameter difference is exactly what the dual matching tolerances in Section 10.3 will not tolerate, which is why you can never pair the two on one hub.

Load Range and Ply Rating

Commercial tires carry a load range letter that corresponds to a historical ply rating — the letter describes the tire's strength and the inflation pressure at which it reaches its rated load, not the literal number of fabric plies:

The sidewall also states the maximum load and the cold inflation pressure at which that load applies — for example, "MAX LOAD SINGLE 6175 LB AT 110 PSI COLD." A tire at less than its stated pressure does not carry its stated load, which is the physical reason underinflation is a structural problem and not just a fuel-economy problem.

[!IMPORTANT] 49 CFR § 393.75(g) makes it illegal to operate a commercial vehicle with a tire carrying more weight than the load marked on its sidewall, except under a special state permit which also limits the vehicle to a maximum of 50 mph. Load range is not advisory.

Speed Rating

Commercial tires may carry a speed symbol or an explicit speed restriction molded into the sidewall. Typical commercial symbols:

49 CFR § 393.75(f) prohibits operating a vehicle with speed-restricted tires rated at 55 mph or less at speeds exceeding that rating. Many severe-service, on/off-road and retreaded tires carry exactly this kind of restriction, and running them at highway speed builds heat faster than the casing can shed it.

Application Marking

The sidewall or the tread designation identifies the intended position: steer/all-position, drive, or trailer. Fitting a drive-position lug tire to a steer axle produces irregular wear and vague steering feel; fitting a free-rolling trailer tire to a drive axle gives away traction. "Verify proper application" on the task list means checking that the tire on the axle is the tire designed for that axle.

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The DOT Code and Tire Age

Every DOT-compliant tire carries a Tire Identification Number molded into the sidewall, beginning with the letters DOT.

   DOT  XX  XX  XXXX  2325
        |   |    |      |
        |   |    |      +-- DATE CODE: week 23 of year 2025
        |   |    +--------- Optional manufacturer's code
        |   +-------------- Tire size code
        +------------------ Plant code (where it was built)

• The last four digits are the date code: the first two are the week of manufacture (01–53) and the last two are the year. A tire stamped 2325 was built in the 23rd week of 2025.
• Tires made before 2000 use a three-digit date code. A three-digit code on a commercial tire means the casing is at least a quarter-century old and should be pulled from service regardless of appearance.
• The full TIN including the date code is molded on only one sidewall. If you cannot find it, check the other side of the tire before concluding it is missing.
• Federal regulations set no mandatory retirement age for a commercial truck tire, but tire manufacturers and fleet maintenance programs commonly specify retirement or at minimum close inspection at six to ten years from the date code, because rubber oxidizes and the casing loses flexibility whether the tire is run or stored.

[!NOTE] Date codes matter most on the units that sit: spare tires, yard trailers, seasonal equipment and dollies. A trailer with abundant tread and a 2014 date code is an aged casing, and age-related belt separation is far more likely than tread wear-out.

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Tread Depth: The Federal Minimums

This is the single most frequently tested tire fact on the ASE T5 exam, and it is a two-number rule set by 49 CFR § 393.75(b) and (c):

Measuring It Correctly

• Measure in a major tread groove, and exclude tie bars, humps and fillets. The regulation says so explicitly, and these raised features sit higher than the groove floor — measuring on one gives a falsely legal reading.
• Measure at several points around the circumference and across the tread width. A tire can be legal at one point and bald at another; irregular wear means the worst point governs.
• Use a calibrated tread depth gauge, not the built-in tread wear indicator (TWI) bars alone. TWI bars are molded at 2/32 inch, so a steer tire that has not yet reached its wear bars can still be illegal at 3/32 inch. This trap — relying on wear bars on a steer axle — appears regularly on the exam.
• Record the measurement. Tread depth across an axle also feeds the dual matching and indirect TPMS issues covered in Section 10.3.

Other Conditions That Fail a Tire Outright

Under 49 CFR § 393.75(a), a commercial vehicle may not be operated on a tire that has:

1. Body ply or belt material exposed through the tread or sidewall
2. Tread or sidewall separation
3. A flat tire or an audible leak
4. A cut that exposes ply or belt material

Add to that the zipper rupture risk covered in Section 10.3: a radial run underinflated has already fatigued its sidewall steel, and it can rupture during re-inflation even after the pressure is restored.

Regrooved, Recapped and Retreaded Restrictions

• Buses may not run regrooved, recapped or retreaded tires on the front wheels at all (§ 393.75(d)).
• Regrooved tires with a load capacity of 2,232 kg (4,920 lb) or greater are prohibited on the front wheels of a truck or truck tractor (§ 393.75(e)).
• Retreads are legal and common on drive and trailer positions; the restriction is about steering positions, where a failure takes away vehicle control rather than just a wheel position.

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Valve Stems, Caps and Tire-to-Rim Matching

Valve Stems

• Inspect stems for cracking at the base, corrosion, bent bodies and wobble. Flex a rubber stem and look for cracks in the rubber; a cracked stem leaks slowly, and a slow leak on a dual is the standard route to a zipper rupture.
• Torque clamp-in stems to specification. Under-torqued stems leak; over-torqued stems crush the grommet and leak worse.
• Support every extension. An unsecured extension hose whips against the wheel, chafes through, and dumps the tire on the highway.

Valve Caps

The cap is not a cosmetic part. It is the secondary pressure seal and the dirt barrier for the valve core. Commercial practice — and TMC recommendation — is a metal or sealing-type cap with a rubber gasket on every valve, including the inner duals. A missing cap lets grit into the core, and a leaking core with no cap is an uncontrolled leak.

Matching Tires and Rims

• Match the tire bead diameter to the rim diameter exactly. A 22.5-inch tire will never safely seat on a 24.5-inch rim, and attempting it — the classic 16-inch tire on a 16.5-inch rim mismatch in lighter service — is a known fatality mechanism during bead seating.
• Match the rim width to the tire's approved rim width range, published by the tire manufacturer. A tire mounted on a rim narrower or wider than its range distorts the casing profile and produces irregular shoulder wear that will not respond to alignment.
• Match construction across an axle. Never mix radial and bias tires on the same axle, and never mix them on a dual pair.
• Match tube-type and tubeless components. Tubeless tires require a tubeless-rated rim with an airtight bead seat; tube-type rims are not sealed at the bead.
• For multi-piece rims, the rim base, side ring and lock ring must all be from the same manufacturer and the same series, as covered in Section 9.3 under OSHA 29 CFR 1910.177.