Yes, cars release methane, but in very small amounts. A typical gasoline car in the U.S. emits roughly 0.016 grams of methane per mile, a tiny fraction of the carbon dioxide pouring out of the same tailpipe. The methane comes from incomplete combustion: not every molecule of fuel burns fully, and some escapes as methane rather than converting to CO2 and water.
How Cars Produce Methane
Gasoline is a mix of hydrocarbons, and under ideal conditions an engine would burn all of them completely. In practice, small pockets of fuel near the cylinder walls and in tight crevices stay cool enough to avoid full combustion. These unburned or partially burned hydrocarbons slip out with the exhaust, and methane is one of them. Engineers sometimes call this “combustion slip.”
The amount depends on how the engine mixes air and fuel. Engines that run with a richer fuel mixture (more fuel relative to air) tend to produce less methane because combustion temperatures are higher and more complete. Leaner mixtures improve fuel economy but can let more methane through. Cold starts also matter: when an engine is still warming up, combustion is less efficient and more unburned fuel escapes before the exhaust system reaches operating temperature.
Why Catalytic Converters Struggle With Methane
Modern cars use catalytic converters to clean up exhaust, breaking down carbon monoxide, nitrogen oxides, and unburned hydrocarbons. But methane is the most chemically stubborn hydrocarbon. Its carbon-hydrogen bonds are unusually strong, requiring high temperatures to break apart on a catalyst surface. Standard catalytic converters don’t become effective against methane until they reach around 350°C (about 660°F), and even then, water vapor in the exhaust interferes with the reaction at temperatures below 450°C.
This means your catalytic converter handles most pollutants well but lets a portion of methane pass through, especially during short trips when the exhaust system never fully heats up. It’s one reason methane is harder to eliminate from tailpipe emissions than other hydrocarbons.
How Much Methane Per Mile
Based on EPA estimates, burning a gallon of gasoline produces about 0.375 grams of methane. For a car averaging 23.4 miles per gallon, that works out to 0.016 grams per mile. To put that in perspective, the same car emits roughly 400 grams of CO2 per mile, so methane is less than one-hundredth of a percent of the exhaust by mass.
That small number is somewhat misleading, though, because methane is a far more potent greenhouse gas than CO2. Over a 100-year period, a ton of methane from fossil fuel sources traps about 29.8 times more heat than a ton of CO2, according to the latest IPCC figures. So while the raw quantity is tiny, each gram of methane carries an outsized climate punch compared to the CO2 alongside it. Still, even after adjusting for potency, the CO2 from your car’s exhaust dominates its total warming impact.
Older Cars Emit More
New vehicles start with low, stable methane emissions. EPA testing shows that methane rates hold steady for roughly the first 20,000 miles, then begin creeping upward as fuel injection systems, sensors, and catalytic converters degrade. The rate of increase depends on the engine type and fuel delivery system. Older carbureted engines from the 1980s started with higher baseline methane (around 0.08 to 0.11 grams per mile) and deteriorated faster than fuel-injected engines, which started closer to 0.02 to 0.04 grams per mile.
Modern port fuel injection systems show the lowest starting emissions and the slowest deterioration. But any car with a failing catalytic converter or misfiring engine will release significantly more methane than one in good working order.
Diesel Engines vs. Gasoline
Conventional diesel engines produce less methane than gasoline engines. Their higher compression ratios create hotter, more complete combustion, leaving fewer unburned hydrocarbons in the exhaust. Diesel vehicles still emit methane, but at lower rates per mile than their gasoline counterparts.
Natural Gas Vehicles Are the Exception
Compressed natural gas (CNG) vehicles are a different story. Because their fuel is roughly 90 to 98 percent methane, any unburned fuel that slips through is almost entirely methane. SAE International testing found that CNG vehicles produce about ten times the methane emissions of comparable gasoline vehicles. The tradeoff: their non-methane hydrocarbon emissions and toxic air pollutants drop to about one-tenth of gasoline levels, and their overall ozone-forming potential is 80 to 90 percent lower.
This creates a genuine climate dilemma. Natural gas burns cleaner in terms of CO2 per unit of energy, but the higher methane leakage from both the tailpipe and the fuel supply chain can offset that advantage. One analysis in Environmental Science & Technology found that switching a heavy-duty truck fleet from diesel to natural gas could actually increase net climate warming for up to 280 years before the CO2 savings catch up to the methane penalty.
The Bigger Picture
Road vehicles are a minor source of methane compared to agriculture, oil and gas production, and landfills, which together account for the vast majority of human-caused methane emissions. The methane from a single car’s tailpipe is genuinely negligible on its own. But with over a billion vehicles on the road worldwide, the collective contribution isn’t zero, particularly from aging fleets in countries without strict emissions testing.
For individual drivers, the practical takeaway is straightforward: keeping your engine and exhaust system well maintained minimizes methane along with every other pollutant. A properly functioning modern car releases so little methane that it barely registers against the CO2 from the same trip.