Cars emit four greenhouse gases: carbon dioxide (CO2), methane, nitrous oxide, and hydrofluorocarbons. Carbon dioxide is by far the largest contributor, with a typical passenger vehicle producing about 4.6 metric tons of it per year. The other three gases come out in much smaller quantities but are far more potent at trapping heat.
Carbon Dioxide: The Primary Emission
Every gallon of gasoline your car burns produces roughly 8,887 grams of CO2, which works out to about 400 grams per mile for an average passenger vehicle. That number can vary significantly depending on fuel economy. A compact sedan getting 35 miles per gallon will produce less CO2 per mile than an SUV getting 20, simply because it burns less fuel to cover the same distance.
CO2 makes up the overwhelming majority of a car’s greenhouse gas output. It forms as a direct, unavoidable byproduct of combustion: the carbon atoms in gasoline combine with oxygen from the air, and CO2 exits the tailpipe. There is no way to reduce this through engine design alone. The only way to cut CO2 from a gasoline car is to burn less fuel.
Methane and Nitrous Oxide
Gasoline vehicles also release small amounts of methane and nitrous oxide from the tailpipe. These gases form during incomplete combustion and through chemical reactions inside the catalytic converter, the device that cleans up exhaust before it leaves the car. Nitrous oxide, for instance, can form when nitric oxide in the exhaust reacts with nitrogen compounds on the catalyst surface, particularly at certain operating temperatures.
Although these emissions are tiny compared to CO2 in terms of volume, they punch above their weight as greenhouse gases. Methane traps about 80 times more heat than CO2 over a 20-year period, and nitrous oxide is roughly 270 times more potent than CO2 over a century. So even small quantities contribute meaningfully to a vehicle’s overall climate impact.
Refrigerant Leaks From Air Conditioning
The fourth greenhouse gas from cars doesn’t come from the tailpipe at all. It leaks from the air conditioning system. Most vehicles use a refrigerant called HFC-134a, which has a global warming potential of 1,430, meaning one gram of it traps as much heat as 1,430 grams of CO2. These leaks happen slowly through seals and hoses over the life of the vehicle, and they happen whether the car is running or not.
Newer vehicles are transitioning to a refrigerant called HFO-1234yf, which has a global warming potential of just 1, essentially the same as CO2. This switch dramatically reduces the climate impact of AC leaks. Federal regulations now offer automakers emissions credits for using lower-impact refrigerants and reducing leakage rates, which has accelerated the transition.
How Driving Habits Affect Emissions
Your actual emissions depend heavily on how and how much you drive. The 4.6 metric tons per year figure assumes about 11,500 miles of annual driving at average fuel economy. If you have a long commute, drive a heavier vehicle, or spend a lot of time in stop-and-go traffic (where engines run less efficiently), your CO2 output will be higher. Highway driving at steady speeds tends to be more fuel-efficient and produces fewer emissions per mile than city driving.
Cold starts matter too. When an engine is cold, it runs richer (burning more fuel relative to air) and the catalytic converter hasn’t reached its operating temperature yet. During those first few minutes, the car produces higher levels of all four greenhouse gases. Short trips where the engine never fully warms up are disproportionately dirty compared to longer drives.
How Electric Vehicles Compare
Electric vehicles produce zero tailpipe emissions, but they aren’t completely emission-free. The electricity used to charge them often comes from power plants that burn fossil fuels, and manufacturing EV batteries requires more energy than building a conventional engine. Research from Argonne National Laboratory found that making an EV with a 300-mile range generates more greenhouse gases during manufacturing than making a comparable gasoline car.
Even so, the math favors EVs over their full lifetime. The EPA notes that total greenhouse gas emissions associated with manufacturing, charging, and driving an EV are typically lower than those from a gasoline car. The gap widens as the electrical grid gets cleaner, and it varies by region. Charging an EV in a state powered mostly by renewables produces far fewer emissions than charging in a state that relies heavily on coal.
Diesel vehicles are worth a brief mention as well. They produce slightly less CO2 per mile than gasoline engines because diesel fuel contains more energy per gallon and diesel engines are more thermally efficient. However, they tend to produce higher levels of nitrous oxide, partially offsetting that advantage.