Do cars produce more CO2 or CO?

Cars produce far more carbon dioxide (CO2) than carbon monoxide (CO) by mass under normal driving conditions. For a typical modern gasoline car, CO2 emissions often range from about 120 to 250 grams per kilometer, while CO emissions are usually a fraction of a gram per kilometer (roughly 0.05 to 1.0 g/km, with the upper end representing regulatory limits rather than typical performance). Diesel cars generally emit even less CO than gasoline cars. The difference in mass is often hundreds of times in favor of CO2.

Why CO2 dominates: the chemistry and technology

CO2 is the primary product of complete combustion of fuel. In a gasoline engine running near its ideal air–fuel ratio, hydrocarbons combine with oxygen to form CO2 and water. CO, by contrast, is a byproduct of incomplete combustion and is most prevalent during cold starts, rich mixtures (such as hard acceleration), or when emissions-control equipment is not functioning properly.

Modern vehicles use three-way catalytic converters that oxidize CO to CO2, sharply reducing CO levels once the catalyst warms up. This is why, even when CO spikes briefly after startup, the cumulative mass of CO2 over a trip remains vastly higher than that of CO.

How much do modern cars emit?

The following figures illustrate typical real-world and regulatory ranges for CO2 and CO emissions for light-duty vehicles. Values vary with driving style, temperature, maintenance, and regulation, but the scale difference between CO2 and CO is consistent.

• CO2 from gasoline vehicles: roughly 120–250 g/km (about 190–400 g/mi), depending on fuel economy. Burning 1 liter of gasoline produces about 2.3 kg of CO2; 1 US gallon produces about 8.9 kg.
• CO from gasoline vehicles: typically well below 1.0 g/km in real-world use; regulatory limits (e.g., Euro 6/Euro 7 gasoline: 1.0 g/km; U.S. Tier 3/LEV III: ~1.0 g/mi) set the ceiling, not the norm.
• Diesel vehicles: CO2 per km comparable to gasoline for similar vehicle size and efficiency; CO typically lower than gasoline (Euro 6/Euro 7 diesel CO limit: 0.5 g/km).
• Hybrids and plug-in hybrids: reduced CO2 per km in proportion to fuel saved; CO remains tightly controlled by the catalyst when the engine runs.
• Battery-electric vehicles: zero tailpipe CO2 and CO; upstream emissions depend on electricity generation mix.

In practice, a gasoline car that emits around 160 g/km of CO2 might emit just 0.1–0.3 g/km of CO. That means CO2 mass is commonly hundreds to thousands of times greater than CO over the same distance.

When does CO spike—and why it still matters

Even though CO mass is far lower than CO2, CO is a highly toxic gas that poses acute health risks at relatively low concentrations, especially in enclosed or poorly ventilated spaces. Certain operating conditions can elevate CO emissions temporarily.

• Cold starts: before the catalytic converter reaches operating temperature, CO oxidation is incomplete.
• Rich mixtures and hard acceleration: extra fuel can produce more CO until the catalyst converts it.
• High altitude or engine/fuel system issues: suboptimal air–fuel mixing can increase CO formation.
• Malfunctioning or aged catalysts: significantly higher CO until repaired.
• Older or small engines without effective catalysts (e.g., some off-road or legacy equipment): much higher CO than modern cars.

These spikes do not change the overall mass balance—CO2 still dominates—but they explain why CO remains a public health concern in specific circumstances, such as idling in enclosed garages.

What the standards say

Regulators focus on both climate and health. CO2 standards target fleet-average efficiency because CO2 is the primary greenhouse gas from tailpipes. Criteria pollutant standards (including CO) protect air quality and health.

• European Union: Euro 6 and the adopted Euro 7 framework keep CO limits for gasoline cars at 1.0 g/km and for diesels at 0.5 g/km, with extensive on-road testing.
• United States: Tier 3/LEV III standards cap CO for light-duty vehicles at roughly 1.0 g/mi on certification cycles, while separate greenhouse gas rules drive down fleet-average CO2.

Real-world testing and on-board diagnostics aim to ensure that vehicles perform well beyond the lab, keeping CO low most of the time while tightening CO2 targets via efficiency and electrification.

Practical ways to keep both CO2 and CO down

Simple habits and maintenance can meaningfully reduce emissions and exposure risk, especially during conditions that elevate CO.

• Maintain the engine and emissions system: fix check-engine lights, replace faulty oxygen sensors, and ensure the catalytic converter is healthy.
• Avoid prolonged idling—particularly in enclosed spaces: reduces CO exposure and saves fuel.
• Drive smoothly and anticipate traffic: gentler acceleration lowers rich-operation events and fuel use.
• Warm up by driving, not idling: the catalyst heats faster under light load.
• Choose more efficient or electrified vehicles when possible: directly reduces tailpipe CO2 and, when electric, eliminates CO at the source.

These measures not only lower climate-impacting CO2 but also minimize the circumstances under which CO can become a localized hazard.

Bottom line

By mass, cars produce much more CO2 than CO—typically hundreds of times more—because complete combustion and catalytic converters favor CO2 formation. CO remains a safety and air-quality concern despite its lower mass, particularly during cold starts, rich operation, or with malfunctioning emission controls.

Summary

Modern cars overwhelmingly emit CO2 rather than CO by mass: roughly 120–250 g/km of CO2 versus generally below 1 g/km of CO, with diesels even lower for CO. Catalytic converters convert most CO to CO2 once warm, so CO is episodic and localized but toxic, while CO2 is continuous and climate-relevant. Good maintenance, smart driving, and cleaner vehicle choices reduce both.

Do cars emit more CO or CO2?

Greenhouse Gas Emissions
Motor vehicle exhausts also produce greenhouse gases that contribute to climate change. The main greenhouse gas produced by vehicles is carbon dioxide (CO2), but they also produce nitrous oxide and methane.

Do cars produce a lot of carbon monoxide?

Gas-powered automobile engines can produce high concentration of carbon monoxide (CO) quickly, overcoming exposed individuals before they realize they are at risk.

What are the top 3 contributors to CO2 emissions?

The majority of this increase has resulted from increased fossil fuel consumption and industrial emissions. Agriculture, deforestation, and other land-use changes have been the second-largest contributors. Emissions of non-CO2 greenhouse gases (CH4, N2O, and F-gases) have also increased significantly since 1850.

Are cars the biggest contributor to climate change?

No, cars are not the single biggest contributor to climate change; rather, the entire transportation sector, which includes cars, trucks, planes, ships, and trains, is a major source of greenhouse gas emissions, making it a significant contributor to climate change. Within the U.S., the transportation sector was the largest source of greenhouse gas (GHG) emissions in 2022, accounting for 28% of the total, with passenger cars and light-duty trucks being the largest contributors within that sector.
 
What is the impact of cars?

• Significant Emissions: Opens in new tabBurning gasoline and diesel in cars and light-duty trucks releases carbon dioxide (CO2) into the atmosphere, a major greenhouse gas responsible for warming the planet. 
• Growing Demand: Opens in new tabThe increased demand for travel, especially by light-duty vehicles like passenger cars and SUVs, has led to a significant rise in transportation emissions from 1990 to 2022. 

What are other major contributors to climate change?

• Electricity Production: Opens in new tabThe electric power sector is another sector that contributes significantly to global emissions. 
• Industry and Agriculture: Opens in new tabOther industrial processes and agricultural practices also release greenhouse gases. 

In summary:
Cars are a major component of the transportation sector, which is the largest source of greenhouse gas emissions in the United States. While cars themselves are a substantial source of these emissions, the issue extends to all forms of transportation, as well as other energy-intensive sectors, all contributing to global climate change.