Tailpipe Emissions

Exhaust emissions standards limit the amounts of key pollutants coming from a vehicle's tailpipe and leaks in its fuel system. All new vehicles for sale in the United States are certified to meet either federal emission standards, set by the U.S. Environmental Protection Agency (EPA), or California standards, set by the California air resources board (carb). Currently the following states have adopted California standards and have implemented them, or soon will: California, Colorado, Delaware, Connecticut, Maine, Maryland, Massachusetts, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, Vermont and Washington. Federal standards apply to all the remaining states.

Today, most cars and light trucks are certified to two standards, one California and one federal. This allows automakers to sell their vehicles nationwide and still comply with both sets of emissions regulations. However, the legal requirements for vehicle certification are complex, and in some instances, vehicles can be certified to a single federal standard nationwide, or to a single California standard sold only in limited areas. In the latter case, these vehicles are most often the very clean “PZEVS” sold either only in California, or to California and the "clean car states" that have adopted California's vehicle regulations.

Current federal standards are the Tier 2 standards introduced in 2004 and fully implemented by 2009, and the Tier 3 standards introduced in 2014 with a phase-in period from 2017 through 2025. These were modeled on California’s LEV II (2004) and LEV III (2012) standards respectively. Tier 3 and LEV III standards bins are based on the sum of non-methane organic gases (NMOG) and nitrogen oxides (NOX) where the least-clean standard is equivalent to the Tier 2 fleet average.

The principal tailpipe standards applicable to today's vehicles are:

Federal Standards:

  • ​Tier 3 bin 160: least-clean tier 3 bin where NMOG+NOX emissions are 160 mg/mi (equivalent to Tier 2 bin 5)
  • ​Tier 3 bins 125, 70, and 50: progressively cleaner standards
  • Tier 3 bin 110 and 85: progressively cleaner transitional bins; equivalent to Tier 2 bins 3 and 4 respectively. Available through model year 2019
  • Tier 3 bin 30: “average” of tier 3 standards. Fleet average NMOG+NOX emissions much reach 30mg/mi by 2025 (Equivalent to Tier 2 Bin 2)
  • Tier 3 bin 20:  progressively cleaner standard
  • Tier 3 bin 0: vehicle with zero tailpipe emissions, equivalent to California ZEV
  • Tier 2 bin 8: least-clean tier 2 bin available today
  • Tier 2 bins 7 and 6: progressively cleaner standards
  • Tier 2 bin 5: "average" of tier 2 standards. NOX emissions levels of all vehicles sold by each automaker must average to the bin 5 NO X level or cleaner.
  • Tier 2 bins 4 through 2: progressively cleaner standards.
  • Tier 2 bin 1: the cleanest federal tier 2 standard. Equivalent to a zero-emission vehicle (ZEV).

California Phase III Low-Emission Vehicle Standards

These are the current low-emission vehicle standards for light duty vehicles; they are more stringent than previous phase II low-emission standards and increases the durability requirements for emission controls. LEV III standards were adopted in 2012 with a phase-in period from 2015 through 2025. Emission categories are based on the sum of NMOG+NOX similar to federal Tier 3 standards. 

  • LEV160: low-emission vehicle; least-clean LEV III standard where NMOG+NOX emission are 160 mg/mi
  • ULEV125, 70, 50: ultra-low-emission vehicle; progressively cleaner LEV III standards
  • SULEV30: super-ultra-low-emission vehicle; Fleet average NMOG+NOX emissions much reach 30 mg/mi by 2025
  • SULEV20: progressively cleaner super-ultra-low-emission vehicle

California Phase II Low-Emission Vehicle Standards

These are the outgoing low-emission vehicle standards; they are more stringent than phase I low-emission vehicle standards previously applicable in California, but will be phased out by model year 2020 when all new vehicles must be certified to new LEV III standards:

  • LEV II: low-emission vehicle, the least stringent of the phase II LEV standards. NOX emissions are one-quarter the level of a LEV I-certified vehicle.
  • ULEV II: ultra-low-emission vehicle, a mid-level phase II LEV standard. Hydrocarbon and carbon monoxide emissions levels are nearly 50% lower than those of a LEV II-certified vehicle.
  • SULEV II: super-ultra-low-emission vehicle, a California standard even tighter than ULEV II, including significantly lower NOX emissions and more durable control systems.
  • PZEV: partial zero-emission vehicle, compliant with the SULEV standard; additionally has near-zero evaporative emissions and a 15-year/150,000-mile warranty on its emission control equipment.
  • ZEV: zero-emission vehicle, a California standard prohibiting any tailpipe emissions.

Prior to the introduction of Tier 2 and LEV II in 2004, vehicles were required to meet the less-stringent Tier 1 (federal) or LEV I (California) standards. These regulatory regimes included the following standards, which are no longer available to new vehicles:

Federal Standards:

  • Tier 1: the former federal (EPA) standard; as of 2005, no longer in use.
  • Tier 1-d: the former federal diesel standard; permits higher NOX emissions than the Tier 1 standard; as of 2005, no longer in use.

California Phase I Low-Emission Vehicle Standards

  • LEV I: low-emission vehicle, about twice as stringent as the now-expired federal tier 1 standard.
  • ULEV I: ultra-low-emission vehicle, a California standard emphasizing low hc emissions.
  • SULEV I: super-ultra-low-emission vehicle, a stronger California phase I standards applicable only to larger passenger trucks (suvs, pickups, etc.).

Automakers also rate their vehicles for fuel economy (miles per gallon—mpg) according to standard EPA tests of simulated city and highway driving. Fuel economy standards apply to manufacturers, rather than to individual vehicles. A manufacturer can sell models of varying fuel economy so long as their average fuel economy meets the standard. Light trucks (pickups, minivans, and sport utilities) have a lower standard than passenger cars and station wagons. A new vehicle's sales sticker shows its city and highway mpg ratings; check these sticker mpg values to help you match a vehicle on the lot to a model listed here on greenercars.com.

Higher fuel economy means lower fuel consumption and, generally, savings on fuel costs and reduced global warming emissions. Even in new vehicles, tailpipe standards don't fully reflect pollution in real-world driving conditions or the pollution associated with production and distribution of the fuel consumed. Therefore, among models meeting a given tailpipe emissions standard, higher fuel economy means lower total emissions of other pollutants as well.

Depending on where you live and how you use your vehicle, you may want to give greater or lesser importance to some factors over others. For this reason ACEEE’s greenercars.org database also provides details on a vehicle's health impacts, its global warming impacts, and its likely fuel costs. Buyers in urban areas, particularly areas with air quality problems, may be especially interested in models that meet tighter tailpipe standards, such as the California LEV III standards, or vehicles already certified to federal Tier 3 standards (i.e., Tier 3 bins 30 through 125) noted above.

Green buyer alert

The pollution coming from a vehicle depends on the standard it meets, how well its emissions controls work, how it is driven and maintained, its fuel consumption, and its fuel quality. Vans, pickup trucks, and sport utilities are classified by the government as light trucks, and as such have less stringent fuel economy standards than passenger cars. As a result, the average light truck pollutes more than the average car. However, the new federal Tier 3 and California LEV III standards bring light truck emissions in line with other light-duty vehicles. 

How to tell which emission standard a vehicle meets

Models can be certified to any of several emission standards, depending on where they are sold and how clean they are. In 2004, eleven federal tier 2 standards (called "bins") went into effect, along with an updated set of California low-emission vehicle (phase II  LEV) standards. With the completion of the Tier 2 phase-in this year, only eight federal bins are available for light-duty vehicles. LEV-certified vehicles are required in Arizona, California, Connecticut, Maine, Maryland, Massachusetts, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Vermont or Washington. In other states, models are typically certified to one of the federal tier 2 bins. Expect to see fewer LEV II and federal Tier 2 certified vehicles as an increasing number of new vehicles are certified to new LEV III and federal Tier 3 standards. Regardless of where you live, however, identifying a vehicle's emission standard is easy. Many automakers now list their cars' and trucks' emission standards under the "vehicle specs" sections of their web sites. Some vehicles have stickers or window decals identifying the certification level. All vehicles have a mandatory under-the-hood label that identifies the emission standard(s)—so while you're standing on the dealer's lot, just pop the hood and have a look.

Greenercars.org scores vehicles using each emission standard to which they are certified. Often, a single vehicle will be certified to both a federal and California standard so that it can be sold throughout the country. We list such models as carrying dual certifications (e.g. "Tier 2 bin 5 / LEV II"), and their Green Scores reflect the cleaner of the standards. If you're still unsure whether you are matching your emission certification up properly, another double-check is to compare the so-called engine family id code. This is a 12-character code (including decimal point) that will have a general format looking something like "7fmxv02.0vzp". This also is required on the under-the-hood label, and can be compared to the engine family code that appears in the "details" page for each model listed in ACEEE’s greenercars.org.

Identifying a vehicle's emission standard is actually a very simple process. The only caveat is that the so-called "clean fuel fleet" certification sometimes listed on the underhood label for regulatory purposes should be ignored, as it will not be as accurate as the California and federal certifications.

In earlier years, greenercars.org identified vehicles that were sold nationwide and met one of the California certifications by placing a star next to their emissions standard listing (e.g., lev*). This proved helpful to consumers who lived outside California but sought out the cleaner, low-emission vehicles. Today, because most vehicles are now being sold carrying both a California and a federal certification, this designation is largely outdated. Consequently, we discontinued this practice beginning with model year 2005 listings.