Real-World Emission Reductions from Passenger Vehicles Over the Past 30 Years (1997)

In 1996, auto and oil industry groups claimed that modern cars emit 96 percent less pollution than 1960s-era cars built before emissions were regulated. These claims were misleading. In fact, 30 years of motor-vehicle pollution control regulations have reduced pollution from the entire US passenger vehicle fleet far less than one would expect. Here's why:

  • A significant gap exists between emission standards and what cars emit in the real world. The primary sources of these excess real-world emissions are malfunctioning emission control equipment, aggressive driving behavior, and air conditioning operation. None of these "off-cycle" emissions are captured in the regulatory test cycle.
  • Total miles driven by all passenger vehicles in the US (cars and light trucks) increased 2.7 times between 1965 and 1995, offsetting substantial amounts of the pollution reductions achieved on a per vehicle basis.
  • The market share of light trucks (pickups, minivans, and SUVs) increased from 15% to 40% between 1970 and 1995. These light trucks have less stringent emission standards than passenger cars, especially for nitrogen oxides. In addition, the number of miles light trucks drive has increased at a much faster rate than the number of miles cars drive: 5.9 times for light trucks versus 2.2 times for cars.

Key Findings

  • The claim that an individual modern car is 96% cleaner for carbon monoxide and hydrocarbons and 90% cleaner for nitrogen oxides than its precontrol-era (1960s) counterpart is overstated. Over its lifetime, the average emission rate for a car is much higher than its emission standard. A modern car will likely emit 4 times more carbon monoxide, 2 times more hydrocarbons, and 3 times more nitrogen oxides (Figure 1).
  • For the entire US passenger vehicle fleet, emissions reductions have been considerably more modest or nonexistent. While emissions of hydrocarbons have been reduced by about two-thirds, emissions of carbon monoxide have dropped only a third, and nitrogen oxide emissions have actually increased more than a fifth over the past 30 years (Figure 2).
  • The light truck fleet's share of US passenger vehicle emissions has increased by 1.7 times for carbon monoxide, 2.3 times for hydrocarbons, and 1.3 times for nitrogen oxides over the past 30 years. For the combined inventory of smog-forming pollutants (hydrocarbons and nitrogen oxides), the share contributed by the light truck fleet has doubled.
  • The passenger vehicle fleet is still the largest single source of carbon monoxide and smog-forming pollutants nationwide. For hydrocarbons and nitrogen oxides separately, it is the second largest source, and its shares of these inventories have not changed appreciably since 1970.
Figure 1. Are Cars Still a Problem?
Summary of Real-World Progress in Reducing Emissions
Over the Past 30 Years

Sources: AAMA claim from Motor Vehicle 1996 Facts and Figures, American Automobile Manufacturers Association for a Tier 1 car. All others, UCS estimates based on data from the US Environmental Protection Agency and the US Department of Transportation.

Note: "Light-duty fleet inventory" (or "passenger vehicle fleet") includes the effect of a 2.7 times increase in total miles driven over this period, the 1995 on-road fleet being composed of mostly Tier 0 vehicles rather than Tier 1 cars, and the increased number of light trucks.


Figure 2. National Emission Inventories for the Passenger Vehicle Fleet for the Period 1965 to 1995
(includes cars, pickup trucks, minivans and sport utility vehicles)

Sources: 1965 inventory is UCS estimate. All other years from National Air Pollution Trends, 1900-1995, US Environmental Protection Agency.


With half of all Americans living in areas that violate national clean air standards, clearly emissions from the passenger vehicle fleet must be reduced. The main strategies that state and federal governments must adopt to reduce pollution from cars and trucks are

  • lower emission standards to treat light trucks and cars equally
  • implement well-run inspection and maintenance programs
  • increase regulatory focus on emission control system durability and off-cycle emissions
  • promote the introduction of truly advanced, intrinsically-clean vehicle technologies that have lifetime, real-world zero or near-zero emissions

We Need Your Support
to Make Change Happen

We can protect consumers, the climate, and our environment from the growing costs and risks of our oil use —but not without you. Your generous support helps develop science-based solutions for a healthy, safe, and sustainable future.