Every tire sold in the United States carries a federally mandated identification number molded into its sidewall that includes the date the tire was manufactured. The last four digits of the DOT Tire Identification Number reveal the week and year of production: a code ending in “2419” means the tire was built during the 24th week of 2019.¹ This date code is not a suggestion or a convenience. It is a regulatory requirement under 49 C.F.R. Part 574, and it exists because tire age is a safety-relevant characteristic that cannot be determined by looking at the tread.²
A tire can have full tread depth and still be dangerous. Rubber compounds degrade over time through a chemical process called thermo-oxidative degradation, in which heat and oxygen cause the internal rubber components to stiffen, crack, and lose their ability to flex under load.³ This degradation occurs whether the tire is in active service or sitting in a warehouse. It accelerates in hot climates and in tires that are mounted and inflated, where the oxygen pressure inside the tire drives the reaction from the inside out.⁴ The result is a tire that looks serviceable on the outside but has lost structural integrity on the inside, making it prone to sudden tread separation, sidewall blowout, or catastrophic delamination at highway speeds.
No federal regulation sets a maximum age limit for tires on commercial vehicles. The federal tire standards under 49 C.F.R. § 393.75 address tread depth, exposed cords, tread separation, and load ratings, but they do not prohibit operating on a tire of any age as long as it meets those condition-based criteria.⁵ This regulatory gap means that a 15-year-old tire with adequate tread depth is legally compliant even though every major tire manufacturer, multiple vehicle manufacturers, and the British Rubber Manufacturers’ Association have recommended that tires be replaced after 6 to 10 years regardless of remaining tread.⁶
How the DOT Date Code Works
The Tire Identification Number, commonly called the DOT number or TIN, is required on every tire by 49 C.F.R. Part 574.⁷ The number is molded into the sidewall and consists of several groupings: a plant code identifying the manufacturing facility, a tire size code, an optional manufacturer descriptor, and the date code. The date code is always the final grouping and consists of four digits.⁸
The first two digits represent the week of manufacture, starting with “01” for the first full calendar week of the year. The second two digits represent the year. A tire stamped “0322” was manufactured in the third week of 2022. A tire stamped “4515” was manufactured in the 45th week of 2015.⁹
Before the year 2000, the date code used only three digits, making it impossible to distinguish between decades. A code ending in “258” could mean the 25th week of 1988 or 1998. The four-digit format was adopted specifically to eliminate this ambiguity after the Firestone/Ford tire recall crisis of 2000 highlighted the importance of tire age in failure analysis.¹⁰
On some tires, the full TIN appears on only one sidewall. The opposite sidewall may display a partial TIN without the date code. This means that to read the manufacture date, an investigator may need to examine the inboard sidewall of a mounted tire, which requires either removing the tire or inspecting the vehicle on a lift. In crash investigations, the date code should be photographed and recorded for every tire on the vehicle, including any spare.
Why Tires Degrade with Age
The science of tire aging is well documented. NHTSA conducted a multi-phase tire aging research program beginning in the early 2000s, collecting tires from service in Phoenix, Arizona, and subjecting them to laboratory analysis alongside new tires of the same models.¹¹ The research established that tires degrade through thermo-oxidative aging: heat and oxygen cause the rubber compounds between the steel belts to stiffen, lose elasticity, and develop microcracks that propagate under the cyclic loading of normal driving.¹²
Key Findings from NHTSA’s Research
The key findings from NHTSA’s research included the following:
- The rubber compound between the belts (the “wedge” and “skim coat” layers) showed systematic increases in hardness and modulus with increasing tire age, meaning the rubber became stiffer and less able to flex.¹³
- The ultimate elongation to break decreased with age, meaning the rubber lost its ability to stretch before failing.¹⁴
- The level of fixed oxygen in the rubber compounds increased with service time, consistent with the mechanism of aerobic oxidation.¹⁵
These changes were observed in tires retrieved from vehicles in Phoenix after as few as three to five years of service.
Time, Not Mileage, Drives Degradation
NHTSA’s research confirmed that time, not mileage, is the primary driver of aging-related degradation.¹⁶ A tire that sits unused on a vehicle or in a warehouse still ages, because the oxygen permeating through the inner liner reacts with the rubber compounds even when the tire is not being driven. Spare tires, which may sit mounted but unused for the life of the vehicle, are subject to the same aging mechanism and showed degradation patterns similar to on-road tires of comparable age.¹⁷
Heat Accelerates the Aging Process
Heat dramatically accelerates the aging process. NHTSA’s data showed that 27% of policyholders from five warm-weather states (Texas, California, Louisiana, Florida, and Arizona) accounted for 77% of all tire-related insurance claims nationwide, and 84% of those claims involved tires older than six years.¹⁸ Tires in hot climates age at rates several times faster than tires in temperate regions, because the rate of the oxidation reaction increases exponentially with temperature.
What Manufacturers Recommend
Despite the absence of a federal age limit, major tire manufacturers have issued their own recommendations. Michelin recommends that tires be thoroughly inspected annually after five years of service and replaced 10 years after their date of manufacture, regardless of remaining tread depth, including spare tires.¹⁹ Bridgestone, Continental, and other manufacturers have issued similar guidance recommending replacement at 10 years from the date of manufacture.²⁰
Some vehicle manufacturers have adopted more aggressive timelines. Ford and Chrysler added warnings to their owner’s manuals in 2006 stating that tires should be replaced after six years of service.²¹ The British Rubber Manufacturers’ Association recommended that unused tires not be put into service if they are over six years old and that all tires be replaced 10 years from the date of manufacture.²²
The six-year threshold is supported by research showing a dramatic increase in tire failure rates after that point. A 1986 DEKRA study examining 146 tread separation failures found a significant increase in failures after six years and concluded that tires should be removed from service after six years regardless of tread depth.²³
What Federal Regulations Do & Do Not Require
For commercial motor vehicles, the tire requirements are found in 49 C.F.R. § 393.75.²⁴
The regulation prohibits operating on any tire that has:
- Exposed body ply or belt material through the tread or sidewall²⁵
- Any tread or sidewall separation
- A flat or audible leak
- A cut exposing ply or belt material
- Steer tires with less than 4/32-inch tread depth; all other tires with less than 2/32-inch²⁶
- Retreaded tires on the front wheels of buses²⁷
- Loads exceeding the weight rating marked on the sidewall²⁸
What the regulation does not include is any reference to tire age. A tire manufactured 12 years ago that still has adequate tread depth, no visible separation, and no exposed cords meets the regulatory standard. The absence of a federal age limit means that carriers can operate on tires of any age as long as the tires pass visual inspection, and that enforcement officers at roadside inspections have no authority to place a tire out of service based on age alone.
This regulatory gap has been the subject of repeated criticism. NHTSA conducted tire aging research, issued a consumer advisory in 2008 warning about the risks of aged tires, and reported its findings to Congress in 2007, but did not promulgate a tire age regulation.²⁹ Several states have considered legislation to regulate tire age at the point of sale or during annual safety inspections, but no state has enacted a mandatory age limit for tires in commercial service.³⁰
How Aged Tires Fail
Aged tires fail through two primary mechanisms, both related to the degradation of the internal rubber compounds.
Tread Separation
Tread separation is the most common and most dangerous aged-tire failure mode. The rubber between the steel belts loses its adhesion to the belt surfaces as the compound stiffens and develops microcracks. Under normal driving loads, the belts begin to separate from each other, and the tread cap peels away from the carcass. The separation may begin at the belt edge and propagate across the width of the tire. The failure can occur suddenly and without warning at highway speeds. When the tread separates from a steer tire, the driver may lose control immediately. When it separates from a drive or trailer tire, the separated tread becomes a large piece of debris that can strike following vehicles or cause the driver to lose directional stability.
Sidewall Blowout
Sidewall blowout is the second failure mode. As the sidewall rubber oxidizes and loses elasticity, it becomes susceptible to sudden failure under the flex loading that occurs with every rotation of the tire. A blowout results in immediate and complete loss of air pressure, causing the vehicle to pull sharply toward the side of the failed tire. On a tractor-trailer combination, a steer-tire blowout at highway speed is among the most dangerous single-point failures a driver can experience.
Both failure modes share a common characteristic: they are often preceded by no visible warning. The internal degradation that causes tread separation occurs between the belts, inside the tire structure, where it cannot be seen during a visual inspection or even during a standard tire pressure check. A tire that passes a pre-trip inspection in the morning can suffer a tread separation that afternoon.
Why Tire Age Matters in Crash Litigation
When a crash involves a tire failure, the manufacture date on the DOT code is among the first pieces of evidence an investigator examines. If the tire is six years old or older, the age of the tire becomes a relevant factor in the failure analysis. If the tire is 10 years old or older, the age becomes a central factor, because every major manufacturer’s own guidance would have recommended replacement before the tire reached that age.
Failure Analysis
The first track is the failure analysis itself: did the tire fail through tread separation or blowout, and is the failure pattern consistent with thermo-oxidative degradation? A forensic tire examiner can evaluate the rubber compounds in the belt area for signs of oxidation, hardening, and adhesion loss that indicate age-related degradation. The condition of the internal rubber, compared against the known aging characteristics documented in NHTSA’s research, can establish whether the tire’s age contributed to the failure.
Liability Analysis
The second track is the liability analysis. The carrier had an obligation under 49 C.F.R. § 396.3 to systematically inspect, repair, and maintain all motor vehicles subject to its control, with all parts in safe and proper operating condition at all times.³¹ A carrier that operated a vehicle on tires it knew or should have known were beyond the manufacturer’s recommended service life has arguably failed to meet that standard. The tire’s DOT date code establishes the age. The manufacturer’s published recommendation establishes the standard. The gap between the two establishes the carrier’s knowledge of the risk.
Under 49 C.F.R. § 396.7, no carrier may operate a vehicle likely to cause an accident or breakdown.³² A 12-year-old tire on a steer axle, operating in a hot climate, at highway speeds, under maximum load, is a tire whose risk of catastrophic failure has been documented by NHTSA research, acknowledged by the manufacturer’s own replacement guidance, and ignored by the carrier that kept it in service.
Sources
[1] 49 C.F.R. § 574.5(b)(4).
[2] 49 C.F.R. Part 574 (Federal Motor Vehicle Safety Standard for Tire Identification and Recordkeeping).
[3] National Highway Traffic Safety Administration, NHTSA Tire Aging Test Development Project Phase 2 — Evaluation of Laboratory Tire Aging Methods (Report No. DOT HS 811 885, Feb. 2014).
[4] Id.
[5] 49 C.F.R. § 393.75.
[6] Safety Research & Strategies, Inc., Two Tire Makers Add Tire Aging Replacement Guidelines for U.S. Market.
[7] 49 C.F.R. Part 574.
[8] 49 C.F.R. § 574.5(b)(4).
[9] Id.
[10] National Highway Traffic Safety Administration, Tire Labeling: Assistance for Consumers in Identifying Older Tires.
[11] James D. MacIsaac Jr., NHTSA Tire Aging Test Development Project Phase 1 — Phoenix, Arizona, Tire Study; Report 1: Laboratory Roadwheel Testing of Light Vehicle Tires as Purchased New and After Retrieval from Service in Phoenix, Arizona, Arizona Memory Project.
[12] National Highway Traffic Safety Administration, NHTSA Tire Aging Test Development Project Phase 2 — Evaluation of Laboratory Tire Aging Methods (Report No. DOT HS 811 885, Feb. 2014).
[13] MacIsaac, supra note 11.
[14] Id.
[15] National Highway Traffic Safety Administration, supra note 3.
[16] National Highway Traffic Safety Administration, Tire Aging: A Summary of NHTSA's Work (Report No. DOT HS 810 799, Aug. 2007).
[17] MacIsaac, supra note 11.
[18] National Highway Traffic Safety Administration, supra note 16.
[19] Michelin USA, When to Replace Tires: Wear, Age, and Safety Signs.
[20] Safety Research & Strategies, Inc., supra note 6.
[21] Sean Kane, Safety Research & Strategies, Inc., Tire Aging and Service Life, presentation at NTSB Tire Safety Symposium (Dec. 9–10, 2014).
[22] Id.
[23] Id. (citing F. Nowakowski, The Aging of Tires — Influence on the Damage Frequency, DEKRA Industry Publication (1986)).
[24] 49 C.F.R. § 393.75.
[25] 49 C.F.R. § 393.75(a).
[26] 49 C.F.R. § 393.75(b)–(c).
[27] 49 C.F.R. § 393.75(d).
[28] 49 C.F.R. § 393.75(g).
[29] National Highway Traffic Safety Administration, supra note 16.
[30] Kane, supra note 21.
[31] 49 C.F.R. § 396.3(a).
[32] 49 C.F.R. § 396.7.