USA: Cars and Light-Duty Trucks

Federal Standards (US EPA)

The most recent emission standards for light-duty vehicles are the Tier 4 standards, that follow the earlier Tier 3, Tier 2, Tier 1, and pre-Tier 1 federal emission regulations.

Two sets of standards had been defined for light-duty vehicles in the Clean Air Act Amendments (CAAA) of 1990: Tier 1 and Tier 2 standards. Once Tier 2 standards were fully phased-in, the EPA adopted Tier 3 and Tier 4 emission regulations. The succession of these regulations can be summarized as follows:

• Tier 1 standards were published as a final rule on June 5, 1991 and phased-in progressively between 1994 and 1997.
• Tier 2 standards were adopted on December 21, 1999, with a phase-in implementation schedule from 2004 to 2009.
• Tier 3 standards were finalized on March 3, 2014 and phased-in from 2017 to 2025.
• Tier 4 standards were adopted on March 20, 2024, with a phase-in schedule from 2027 through 2033.

Tier 1 standards applied to all new light-duty vehicles (LDV) of less than 8500 lb gross vehicle weight rating (GVWR). The Tier 2 rule extended the applicability of the light-duty emission standards to medium-duty passenger vehicles (MDPV) with GVWR between 8500 and 10,000 lbs. Tier 3 and Tier 4 regulations additionally include emission standards for chassis-certified heavy-duty vehicles up to 14,000 lbs (Class 2b and Class 3).

The successive tiers of emission regulations do not begin with a sharp cut-off date. Rather, each new tier of emission standards is phased-in over a number of years. During the phase-in period, manufacturers are required to certify an increasing percentage of their new vehicle fleet to the new standards, with the remaining vehicles still certified to the preceding tier of emission regulations.

The US emission standards establish (a set of) certification emission limits applicable to each certified vehicle, as well as fleet average standards. The fleet average standards—for NOx at the Tier 1 and Tier 2 stages and for NMOG+NOx at Tier 3 and Tier 4—distinguish the US emission regulations from other light-duty emission standards for criteria pollutants across the world, which typically include only certification/type approval limits.

California Standards (CARB)

California emission regulations for light duty vehicles are designated Low Emission Vehicle (LEV) standards. The succession of California standards—including the LEV I, LEV II, LEV III, and LEV IV stages—has closely followed (or, more accurately, preceded) the respective federal standards.

Historically, California standards have been more stringent than the federal ones. At the LEV III and EPA Tier 3 stages, several provisions of the two sets of standards became more closely aligned. However, many differences remain between the LEV IV and EPA Tier 4 standards in regards to the implementation schedules as well as other important regulatory requirements.

Under Section 177 of the Clean Air Act, other states have a choice to either follow the federal regulations or to adopt California emission standards. A number of states who needed emission standards more stringent than the federal regulations in order to meet their air quality targets have adopted California standards, including:

• Advanced Clean Cars II regulation (2026)—The ACC II package includes LEV IV criteria pollutant standards and zero emission vehicle (ZEV) requirements for model years 2026 and later.
• LEV Criteria Pollutant standards (1992)
• LEV Greenhouse Gas standards (2009)
• ZEV standards (1990)

States that adopted California standards—so-called Section 177 States—are listed in the following table [5189].