What carbon emissions from vehicles actually measure

Carbon emissions from vehicles are the amount of carbon dioxide (CO2) your car releases into the air when you drive. Every gallon of gasoline burned produces roughly 20 pounds of CO2. The total emissions depend on how much fuel your vehicle uses and how far you drive — a car that gets 25 miles per gallon produces less CO2 per mile than one that gets 15 miles per gallon, even if both are burning the same type of fuel.

When people talk about "carbon emissions by year," they usually mean one of two things: the total CO2 that all vehicles on the road produced in a given year, or the emissions rating assigned to your specific vehicle model for that year. Both numbers matter for different reasons. The national total shows whether transportation is becoming cleaner or dirtier overall. Your vehicle's rating shows how it compares to other cars made in the same year and helps determine whether it will pass an emissions test in your state.

Key Takeaways

  • Vehicle emissions standards have become stricter almost every year since the 1970s, meaning new cars produce far less CO2 than older ones.
  • Your vehicle's emissions rating depends on the year it was made, not how old it is now — a 2015 car has a 2015 emissions standard regardless of when you buy it.
  • Total U.S. transportation emissions have risen over the past two decades even as individual vehicles became cleaner, because more people drive and drive farther.
  • Emissions tests in your state measure pollutants like nitrogen oxides and particulates, not just carbon dioxide, and standards vary by state.
  • Electric vehicles produce zero tailpipe emissions, but the total carbon impact depends on how the electricity in your region is generated.

How emissions standards have tightened since the 1970s

The first federal emissions standards took effect in 1975, after the Clean Air Act was passed. Cars made that year were allowed to produce roughly 15 grams of CO2 per mile. By 2010, that limit had dropped to about 250 grams per mile for the average new car sold. By 2023, the limit was around 200 grams per mile, and it continues to drop.

The reason the numbers look backwards — lower grams per mile is stricter — is that the standard measures how much CO2 a car produces for every mile driven. A car that produces 200 grams per mile is cleaner than one that produces 250 grams per mile. Manufacturers have met these tightening standards by improving engine efficiency, reducing vehicle weight, and adding technologies like turbocharging and hybrid systems.

Each state can either follow the federal standard set by the EPA or adopt California's standard, which is stricter. About a dozen states follow California's rules. This means a 2023 car sold in California must be cleaner than the same model sold in a state following federal standards only.

Why total emissions keep rising even as cars get cleaner

Individual vehicles are producing less CO2 than they did 20 years ago, but total transportation emissions in the United States have risen. This happens because more people drive, people drive longer distances, and trucks and SUVs — which produce more emissions than sedans — have become the dominant vehicle type on the road.

Between 2000 and 2020, the number of registered vehicles in the U.S. grew by roughly 40 million. At the same time, the average distance driven per vehicle per year increased. Even though a 2020 car produces less CO2 per mile than a 2000 car, the total amount of CO2 from all vehicles combined went up because there are so many more of them and they are driven so much more.

This gap between individual vehicle improvements and total emissions growth is why emissions standards keep getting stricter — regulators are trying to make each new car clean enough to offset the growth in driving.

How your vehicle's emissions rating is determined

Your car's emissions rating is based on the year it was manufactured, not the current year or how old the vehicle is. A 2010 Honda Civic has a 2010 emissions standard whether you bought it new in 2010 or used in 2024. The rating is determined by the manufacturer during testing and is tied to that model year's federal or state standards.

The EPA tests new vehicles in a lab using a standardized driving cycle that simulates city and highway driving. The test measures how much CO2 the vehicle produces over that cycle and calculates grams per mile. Manufacturers must report this number, and it appears on the window sticker of every new car sold.

When you take an emissions test at an inspection station in your state, the technician is not measuring CO2 directly in most cases. Instead, they measure nitrogen oxides (NOx), particulate matter, and other pollutants that harm air quality. These tests check whether your vehicle is still performing within the standard for its model year, accounting for wear and tear.

The difference between CO2 and other emissions your car produces

Carbon dioxide is a greenhouse gas that contributes to climate change, but it is not the pollutant that fails most vehicles on emissions tests. Nitrogen oxides, volatile organic compounds, and particulate matter are the main pollutants that state inspection programs measure. These substances damage air quality and cause respiratory problems in people who breathe them.

A car can produce very little CO2 but still fail an emissions test if its engine is not tuned properly and it is producing too much NOx or particulates. This is why a vehicle that runs fine and gets good fuel economy can still fail inspection — the test is checking for pollutants that affect local air quality, not just climate impact.

Diesel vehicles produce different emissions profiles than gasoline vehicles. Diesel engines typically produce more nitrogen oxides and particulates but often produce less CO2 per mile because diesel fuel contains more energy per gallon. Some states have separate emissions standards for diesel vehicles.

What electric vehicles mean for emissions by year

Electric vehicles produce zero tailpipe emissions — no CO2, no NOx, no particulates come out of the vehicle itself. However, the total carbon impact of an electric vehicle depends on how the electricity in your region is generated. If your state's power grid relies heavily on coal, an electric vehicle is cleaner than a gas car but not as clean as it would be in a state with more renewable energy.

As electric vehicles make up a larger share of new car sales each year, the average emissions of all new vehicles sold drops significantly. In 2023, electric vehicles accounted for about 14% of new car sales in the U.S., and that percentage is rising. This shift is one of the main reasons that federal emissions standards are becoming stricter — regulators are accounting for the fact that future fleets will include many zero-emission vehicles.

If you own an electric vehicle, you will not have to take an emissions test in most states, since there are no tailpipe emissions to measure. A few states still require an inspection to check the vehicle's safety systems, but not its emissions.

How to find your vehicle's emissions rating

The easiest place to find your vehicle's emissions rating is the window sticker that came with the car when it was new. This sticker shows the EPA's estimated fuel economy and the CO2 emissions in grams per mile. If you bought the car used and do not have the sticker, you can look up the rating on the EPA's website using the vehicle's year, make, model, and engine size.

You can also find emissions information on the manufacturer's website or in your owner's manual. The rating will be listed under fuel economy or environmental information. Keep in mind that the rating is an estimate based on standardized testing — your actual emissions will vary depending on how you drive, road conditions, and how well your vehicle is maintained.

If you need to know whether your vehicle will pass an emissions test in your state, contact your state's environmental agency or the inspection station where you plan to test. They can tell you what the current standard is for your vehicle's model year and what pollutants they measure.

Frequently Asked Questions

Does a newer car always produce less emissions than an older one?

Yes, in terms of the emissions standard it was built to meet. A 2023 car meets a stricter standard than a 2010 car. However, if an older car is well-maintained and a newer car is not, the older car might produce less pollution on an actual test. The standard is what matters for inspection purposes.

Can I reduce my vehicle's emissions by changing my driving habits?

You can reduce the total amount of CO2 you produce by driving less, combining trips, and maintaining steady speeds. However, you cannot change your vehicle's emissions rating or make it pass an inspection test by driving differently. The rating is fixed to the model year.

What happens if my car fails an emissions test?

You will not be able to renew your registration until you repair the vehicle and pass a retest. Common fixes include replacing a faulty oxygen sensor, fixing a vacuum leak, or replacing a clogged fuel filter. If repairs are expensive, you may be able to get a waiver in some states if you have made a good-faith repair attempt.

Are emissions standards the same in every state?

No. Most states follow the federal EPA standard, but California and about a dozen other states have adopted stricter standards. A few states have no emissions testing program at all. Check your state's environmental agency website to find out what applies where you live.

Does my electric vehicle need an emissions test?

Most states do not require emissions testing for electric vehicles since they produce no tailpipe emissions. Some states still require a safety inspection, but not an emissions test. Check your state's requirements to be sure.