GHG emissions are the gases your vehicle releases that trap heat in the atmosphere and contribute to climate change

GHG stands for greenhouse gas. When your car burns fuel, it produces carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) — all gases that trap warmth in the Earth's atmosphere. The EPA measures these emissions in grams per mile and uses a single number called the "CO2 equivalent" to compare how much heat-trapping power a vehicle produces, regardless of which specific gases it releases.

Your vehicle's GHG rating appears on the window sticker when you buy or lease a car, and it also shows up on your emissions test results if your state requires testing. A lower number means the vehicle produces less heat-trapping gas per mile driven. The rating helps you understand the climate impact of the vehicle you own or are considering, and it's one of the metrics regulators use to track whether the vehicle fleet is becoming cleaner over time.

Key Takeaways

  • GHG emissions are carbon dioxide, methane, and nitrous oxide that escape from your tailpipe and trap heat in the atmosphere.
  • The EPA converts all three gases into a single "CO2 equivalent" number so you can compare vehicles on one scale.
  • Your vehicle's GHG rating appears on the Monroney label (window sticker) and on emissions test reports in states that require testing.
  • Lower GHG numbers mean the vehicle produces less climate-warming gas per mile, which saves fuel and reduces your environmental footprint.

How the EPA measures GHG emissions from cars

The EPA tests vehicles in a laboratory using a standardized driving cycle that mimics city and highway driving. The test measures how much CO2, methane, and nitrous oxide the vehicle produces over that cycle, then converts all three into grams of CO2 equivalent per mile. This conversion accounts for the fact that methane and nitrous oxide trap more heat than CO2, so a small amount of either one counts as more "equivalent" CO2.

The result is a single number — for example, 250 grams of CO2 equivalent per mile — that lets you compare any two vehicles directly. A sedan rated at 200 grams per mile produces less heat-trapping gas than an SUV rated at 300 grams per mile, even if one produces more methane and the other produces more CO2. The EPA publishes these ratings for every new vehicle sold in the United States, and manufacturers must meet fleet-wide targets that improve each year.

Where you see GHG emissions information

When you buy or lease a new car, the Monroney label (the window sticker) displays the vehicle's estimated annual fuel cost and its GHG emissions rating in grams per mile. This label is required by federal law and gives you a quick way to compare the climate impact of different models before you decide.

If your state requires emissions testing, your test report will also show the vehicle's GHG emissions. Some states test only for smog-forming pollutants like nitrogen oxides and particulate matter, while others include GHG measurements. Your test results tell you whether your specific vehicle is producing emissions within the legal limits for your state, and the GHG number shows how much climate-warming gas it released during the test cycle.

Why GHG emissions matter for fuel economy

GHG emissions and fuel economy are closely linked. A vehicle that burns less fuel produces fewer greenhouse gases, because less fuel burned means less CO2 released. The EPA rates vehicles on both metrics — miles per gallon (MPG) and grams of CO2 equivalent per mile — and they move together. A car that gets 40 MPG will almost always have a lower GHG rating than one that gets 25 MPG.

This connection means that choosing a vehicle with lower GHG emissions saves you money at the pump while also reducing your climate impact. Over the life of a vehicle, the fuel savings can be substantial. A car rated at 200 grams per mile will cost less to fuel than one rated at 350 grams per mile, assuming you drive the same distance.

GHG emissions vs. smog-forming pollutants

GHG emissions and smog-forming pollutants are different things, and your vehicle produces both. Smog-forming pollutants like nitrogen oxides (NOx) and particulate matter (PM) damage air quality and cause respiratory problems in people who breathe them. GHG emissions trap heat globally and contribute to climate change. A vehicle can pass a smog test — meaning it produces low levels of NOx and PM — but still have a high GHG rating if it burns a lot of fuel.

Emissions testing in your state focuses on smog-forming pollutants because they affect local air quality and public health. GHG emissions are tracked by the EPA at the national level to measure progress toward climate goals. Both matter, but they address different problems: one is about the air you breathe today, and the other is about the climate you live in tomorrow.

How vehicle type affects GHG emissions

Larger, heavier vehicles produce more GHG emissions than smaller, lighter ones, because the engine has to burn more fuel to move the extra weight. A full-size pickup truck typically has a GHG rating of 400 to 500 grams per mile, while a compact sedan might be rated at 200 to 250 grams per mile. Electric vehicles produce zero tailpipe GHG emissions, though the electricity they use may come from power plants that produce emissions.

Hybrid vehicles — which use both a gas engine and an electric motor — produce lower GHG emissions than conventional gas vehicles of the same size, because the electric motor reduces fuel consumption. Plug-in hybrids can produce even lower emissions if you charge them regularly and drive short distances on battery power. The vehicle type you choose has one of the biggest effects on your GHG emissions over time.

Understanding your vehicle's GHG rating

Your vehicle's GHG rating is an estimate based on laboratory testing, not a measurement of what your specific car actually produces. Real-world driving produces different emissions depending on how you drive, the condition of your vehicle, and the fuel you use. A vehicle rated at 250 grams per mile might produce 200 grams in highway driving or 300 grams in stop-and-go city traffic.

You can reduce your vehicle's real-world GHG emissions by keeping it well-maintained, driving smoothly without rapid acceleration, maintaining proper tire pressure, and removing excess weight from the trunk. These habits improve fuel economy and lower emissions at the same time. If your vehicle fails an emissions test, the problem is usually with smog-forming pollutants, not GHG — but fixing the issue that caused the failure will also improve fuel economy and lower GHG emissions.

Frequently Asked Questions

What's the difference between GHG and CO2?

CO2 is one type of greenhouse gas. GHG is the broader category that includes CO2, methane, and nitrous oxide. The EPA converts all three into "CO2 equivalent" so you can compare them on one scale, even though methane and nitrous oxide trap more heat per molecule than CO2 does.

Does a lower GHG rating mean my car will pass emissions testing?

Not necessarily. Emissions testing in most states measures smog-forming pollutants like nitrogen oxides and particulate matter, not GHG emissions. A vehicle can have a low GHG rating and still fail a smog test, or vice versa. The two measure different things.

Can I reduce my vehicle's GHG emissions by changing how I drive?

Yes. Smooth acceleration, steady speeds, proper tire pressure, and regular maintenance all improve fuel economy and lower real-world GHG emissions. However, your vehicle's EPA-rated GHG number stays the same — that rating is based on laboratory testing and doesn't change based on individual driving habits.

Do electric vehicles have zero GHG emissions?

Electric vehicles produce zero tailpipe GHG emissions, but the electricity they use may come from power plants that produce emissions. The total climate impact depends on how clean your local power grid is. In regions with renewable energy, electric vehicles have much lower lifetime GHG emissions than gas vehicles.

Why does the EPA use CO2 equivalent instead of just measuring CO2?

Methane and nitrous oxide trap much more heat than CO2, so a small amount of either one has a larger climate impact. Using CO2 equivalent lets the EPA account for this difference and give you a single number that reflects the true heat-trapping power of all the gases your vehicle produces.