What “CO2” Means on a Car

CO2 on a car refers to carbon dioxide associated with the vehicle—most commonly its official tailpipe CO2 emissions rating, shown as grams per kilometer (g/km) or grams per mile (g/mi). That number indicates how much CO2 the car emits while driving and is used to compare environmental impact, estimate fuel costs, and determine taxes or incentives. It can also appear on labels for air-conditioning refrigerant (R‑744, which is CO2) or on cabin-air sensors that display CO2 levels inside the vehicle.

CO2: What It Is and Why It Matters

Carbon dioxide is a colorless greenhouse gas produced when carbon-based fuels—gasoline, diesel, natural gas, and biofuels—are burned. In road transport, most CO2 comes from the vehicle’s tailpipe and scales almost directly with the amount of fuel burned. Higher CO2 means higher fuel use. Regulators track CO2 because it contributes to climate change and because it is a simple, consistent indicator of fuel efficiency. Battery-electric vehicles have zero tailpipe CO2, though their lifecycle emissions depend on how electricity and materials are produced.

Where You’ll See CO2 on Car Listings and Labels

Automakers, dealers, and regulators display CO2 figures to make environmental impact and running costs easier to understand. Here are the most common places you’ll encounter them and what they mean.

• Specs sheets and brochures: A “CO2 (combined)” rating in g/km (Europe/UK) or g/mi (U.S.) summarizing typical mixed driving emissions.
• Window/Monroney labels: U.S. EPA labels show fuel economy, greenhouse-gas ratings, and grams of CO2 per mile; UK/EU labels show WLTP fuel economy and CO2 g/km.
• Taxes and incentives: Many countries use CO2 bands to set registration taxes, congestion charges, or company-car Benefit-in-Kind rates. Lower CO2 generally means lower tax.
• Fleet and policy targets: Manufacturers’ compliance targets and city access rules often hinge on certified CO2 values.
• Apps and telematics: Some vehicles and insurance or fleet apps estimate your trip CO2 based on fuel or electricity use.

These labels let shoppers compare different vehicles on an equal basis and help policymakers nudge markets toward more efficient or zero-emission models.

How CO2 From Driving Is Measured

Official CO2 ratings come from standardized lab tests designed to represent typical driving. Real-world results vary with speed, temperature, terrain, payload, and driving style, but the certification numbers give a consistent baseline for comparisons and regulation.

Europe and UK

CO2 is certified on the WLTP (Worldwide Harmonised Light Vehicles Test Procedure), which replaced the older, more optimistic NEDC. WLTP is paired with on-road RDE tests for pollutants. Labels display urban, extra-urban, and combined figures; taxes and compliance generally use the combined value.

United States

The EPA uses laboratory drive cycles (2-cycle and 5-cycle methods) to produce city, highway, and combined fuel economy and corresponding CO2. Labels also include a 1–10 greenhouse-gas rating; the lower the CO2 per mile, the better the score.

Real-World vs. Ratings

Real-world CO2 can be higher than the label value in cold weather, at high speeds, or with heavy loads. Conversely, gentle driving and warm conditions can beat the label. The label remains the best apples-to-apples comparator across models.

How Much CO2 Different Fuels Produce

Tailpipe CO2 is fundamentally tied to carbon content. The figures below are widely used regulatory values for combustion emissions; full “lifecycle” impacts can be higher or lower depending on production and supply chains.

• Gasoline: About 8.89 kg CO2 per U.S. gallon (≈2.35 kg per liter). A gasoline car’s CO2 per km rises nearly linearly with fuel use.
• Diesel: About 10.16 kg CO2 per U.S. gallon (≈2.68 kg per liter). Diesel has more carbon per liter but often better mpg, so net CO2 depends on the vehicle.
• Ethanol blends (e.g., E10/E85): Lower carbon per liter at the tailpipe; lifecycle CO2 depends on how the biofuel is produced.
• CNG (compressed natural gas): Lower CO2 per unit energy than gasoline; methane leakage upstream can affect lifecycle impact.
• Electricity: Battery-electric vehicles emit 0 g/km at the tailpipe; total CO2 depends on the charging mix (renewables vs. fossil grid) and manufacturing.
• Hydrogen fuel cell: Tailpipe emits water only; lifecycle CO2 depends on hydrogen production (green via electrolysis vs. grey/blue from natural gas).

Because CO2 scales with fuel burned, improvements in efficiency directly reduce CO2, regardless of the fuel type.

What Affects a Car’s CO2 Rating

Two vehicles with similar size can have very different CO2 numbers. These factors explain why.

• Vehicle mass and aerodynamics: Heavier, boxier vehicles use more energy per mile.
• Powertrain efficiency: Engine design, transmissions, hybrids, and plug-in hybrids improve efficiency.
• Tires and rolling resistance: Efficient tires lower energy losses.
• Speed and driving style: High speeds, rapid acceleration, and hard braking increase consumption.
• Temperature and accessories: Cold weather, HVAC, and roof racks can raise CO2.
• Payload and towing: Extra weight or trailers significantly increase emissions.

Manufacturers optimize these variables for certification tests, while drivers influence many of them every day.

Ways to Reduce Your Car-Related CO2

You can cut emissions through what you drive and how you drive. The options below range from one-time choices to everyday habits.

1. Choose a lower-CO2 vehicle: EVs or efficient hybrids can dramatically reduce tailpipe CO2.
2. Right-size the car: Smaller, lighter models typically emit less.
3. Drive smoothly: Anticipate traffic, avoid hard acceleration, and keep steady speeds.
4. Mind your speed: Aerodynamic drag rises quickly above highway speeds.
5. Check tires: Proper pressure and low-rolling-resistance tires save fuel.
6. Lighten the load: Remove unnecessary cargo and roof racks when not in use.
7. Use eco modes and preconditioning: Especially effective in hybrids and EVs.
8. Plan trips: Combine errands, avoid peak congestion, and use navigation that optimizes efficiency.
9. Maintain the vehicle: Timely servicing keeps efficiency and emissions in check.
10. Charge smart if you have an EV: Use low-carbon electricity where possible (off-peak or renewable sources).

Even without changing vehicles, thoughtful driving and maintenance can yield meaningful reductions in CO2 and fuel costs.

Other Times “CO2” Appears on a Car

Not every “CO2” label refers to tailpipe emissions. You may see it in other, unrelated contexts.

• Air-conditioning refrigerant: Some systems use R‑744 (CO2) refrigerant; labels under the hood note the type and quantity.
• Cabin-air quality displays: A few models or aftermarket sensors show cabin CO2 in parts per million (ppm). High levels can cause drowsiness; use fresh air to reduce.
• Fire safety gear: Motorsport or garage extinguishers may be labeled CO2, indicating the extinguishing agent.

These labels don’t indicate the car’s environmental rating; they’re about HVAC technology, air quality, or safety equipment.

Quick Reference and Conversions

Because CO2 is tied to fuel burned, simple rules of thumb help you translate between fuel economy and CO2.

• Gasoline: CO2 g/km ≈ 23.1 × (liters/100 km). Example: 6.0 L/100 km ≈ 139 g/km.
• Diesel: CO2 g/km ≈ 26.8 × (liters/100 km).
• U.S. mpg to g/km (gasoline): CO2 g/km ≈ 5525 ÷ mpg. Example: 40 mpg ≈ 138 g/km.
• Kilometers to miles: g/mi = g/km × 1.609; g/km = g/mi ÷ 1.609.

These estimates align closely with official label values and are useful for quick comparisons across markets.

Summary

CO2 on a car primarily refers to its rated tailpipe carbon dioxide emissions, a direct proxy for fuel consumption and a key metric for taxes, incentives, and environmental impact. Ratings are measured on standardized tests (WLTP in Europe/UK, EPA cycles in the U.S.) and typically appear on spec sheets and window labels. CO2 depends on fuel type, vehicle design, and how you drive; EVs are zero at the tailpipe but have lifecycle emissions tied to the electricity mix. You may also see “CO2” on air-conditioning refrigerant labels (R‑744) or cabin-air sensors, which are unrelated to the emissions rating.

How does CO2 get in your car?

External air pollution: In areas with heavy traffic or high pollution levels, there might be an increased presence of CO2 from the surrounding environment. This can seep into the car, especially if the windows are open or the ventilation system is not effective at filtering the outside air.

What causes high CO2 emissions in cars?

High CO2 emissions in cars are primarily caused by the combustion of fossil fuels, but can be increased by poor engine health (like faulty sensors or dirty filters), inefficient driving behavior, and vehicle factors such as heavy weight and lower fuel efficiency. Factors that lead to incomplete fuel combustion, such as a rich air-fuel mixture, can also increase emissions. 
Fundamental Cause 

• Fuel Combustion: The burning of gasoline and diesel fuel is the largest source of carbon dioxide from vehicles. Every gallon of gasoline burned releases carbon dioxide into the atmosphere.

Engine-Related Factors

• Engine Components: Opens in new tabFaulty components like a bad oxygen sensor, a clogged air filter, or a dirty fuel injector can lead to an improper air-fuel mixture, causing incomplete combustion and increased CO2. 
• Engine Misfires: Opens in new tabWorn-out spark plugs or other ignition problems can cause engine misfires, which also contribute to higher emissions. 
• Rich Air-Fuel Mixture: Opens in new tabAn imbalance of fuel and air, often caused by a faulty sensor or a clogged filter, results in too much fuel, leading to increased emissions. 

Vehicle and Driving Factors

• Vehicle Weight: Opens in new tabHeavier vehicles require more energy to move, leading to lower fuel efficiency and, consequently, higher emissions. 
• Driving Behavior: Opens in new tabAggressive driving, such as rapid acceleration and braking, reduces fuel efficiency and increases CO2 emissions. 
• Fuel Efficiency: Opens in new tabA vehicle with lower fuel efficiency will burn more fuel over time, leading to greater CO2 emissions. 

How do you fix your CO2 levels?

To fix high indoor CO2 levels, increase ventilation by opening windows or using fans to bring in fresh air, and use CO2 monitors to track your home’s air quality. Other methods include using exhaust fans when cooking, avoiding indoor smoking, and ensuring fuel-burning appliances are properly vented. For a broader impact on atmospheric CO2, actions like reducing your carbon footprint, investing in renewable energy, and planting trees can help. 
For Indoor Air Quality

• Increase ventilation: Opens in new tabOpen windows and doors to allow fresh air to circulate, which is the most effective way to lower indoor CO2 levels. 
• Use mechanical ventilation: Opens in new tabEnsure your heating, ventilation, and air conditioning (HVAC) system is running optimally. Consider installing a Demand-Controlled Ventilation (DCV) system, which adjusts ventilation based on real-time CO2 sensors. 
• Use exhaust fans: Opens in new tabTurn on kitchen and bathroom exhaust fans to remove CO2 generated during cooking or other activities. 
• Control indoor sources: Opens in new tabDo not smoke indoors and ensure that any fuel-burning appliances (like gas stoves or unvented heaters) are in good working order and properly vented to the outside. 
• Monitor CO2 levels: Opens in new tabUse a CO2 monitor to track levels and determine if your efforts are effective. Aim for levels below 1,000 ppm for good air quality. 
• Reduce occupancy density: Opens in new tabFewer people in a space means CO2 will rise more slowly. 

For Broader, Long-Term CO2 Reduction (Carbon Footprint)

• Conserve energy: Improve your home’s weatherization and switch to energy-efficient appliances. 
• Use renewable energy: Tap into renewable power sources like solar and wind energy. 
• Opt for low-carbon transport: Choose public transportation, cycling, or electric vehicles over gasoline-powered cars. 
• Support natural carbon capture: Plant trees and support initiatives that manage and expand woodlands, which naturally absorb CO2. 
• Improve soil health: Agricultural practices that improve soil health can help capture carbon. 

What does CO2 mean on a car?

carbon dioxide
Every litre of diesel fuel burned will produce 2.6kg of carbon dioxide whilst petrol produces about 2.3kg of carbon dioxide per litre. If drivers spend more time making longer journeys on motorways for example, they should consider a diesel engine for its fuel efficiency and its lower C02 emissions.