Why this number is unusually solid
Most automotive figures are test results. Fuel economy comes from a defined drive cycle, emissions of regulated pollutants come from a laboratory, and both can differ from the road by a lot. Carbon dioxide is different. Gasoline is roughly 87 percent carbon by mass, and combustion converts essentially all of that carbon to CO2, so the CO2 from a gallon of gasoline is a stoichiometric consequence rather than an observation. The commonly used figure is 8,887 grams per gallon; diesel, being denser and more carbon-rich per gallon, comes to about 10,180.
The practical consequence is that fuel economy is the only variable. There is no engine technology, no aftertreatment and no calibration that changes CO2 per gallon burned. Anything that raises MPG lowers CO2 in exact proportion, and anything that lowers MPG raises it. A car at 12,000 miles a year and 25 MPG burns 480 gallons and emits about 4,266 kilograms of CO2, and the only way to change that figure is to change the gallons.
Diesel per gallon against diesel per mile
Diesel emits about 15 percent more CO2 per gallon than gasoline, which sounds like a straightforward penalty and is not. Diesel engines are generally more efficient, so a diesel version of a given car usually returns higher MPG, and the two effects largely offset. Whether the diesel comes out ahead per mile depends on the size of the MPG advantage in your actual driving.
| Case | g CO2 per mile | Comment |
|---|---|---|
| Gasoline at 25 MPG | 355 | 8,887 ÷ 25 |
| Diesel at 25 MPG | 407 | Same economy, more carbon per gallon — diesel loses |
| Diesel at 30 MPG | 339 | A 20% economy advantage puts it slightly ahead |
| Diesel at 35 MPG | 291 | A large advantage, and the diesel is clearly ahead |
None of which addresses the other emissions from diesel combustion, which is where regulation has focused and where the arguments about diesel actually live. This page is about carbon dioxide alone.
The electric comparison is a comparison with your grid
An electric car has no tailpipe emissions, so its operating CO2 is whatever was emitted making the electricity it consumed. That makes the answer regional in a way no other automotive figure is. The calculator therefore asks for a grid emission factor rather than assuming one, and prints three things: your result at the factor you entered, the result at each end of the range US regions actually span, and the grid factor at which an electric car would match the combustion car you described.
That last figure is usually the most informative. For a 25 MPG gasoline car and an electric car at 3.5 miles per kWh, the break-even grid intensity is around 2.7 pounds of CO2 per kWh, which is above anything a US regional grid runs at — the electric car wins on operating emissions everywhere, but by anything from a factor of two to a factor of ten. For a 50 MPG hybrid the break-even is half that, and the margin narrows considerably on a high-carbon grid.
Reducing the number without changing the car
Since CO2 is proportional to gallons burned, everything that improves real-world economy reduces it by the same percentage, and the largest lever is usually the one nobody wants to hear about: miles not driven. Beyond that, the ordinary list applies and its effects are measurable rather than mystical. Correct tire pressure, a removed roof box, less idling, moderate highway speeds and combined trips instead of several cold starts each recover a few percent, and a few percent of 480 gallons is real. What none of them do is change the grams per gallon, which is why this page has no field for driving style: whatever it does, it does through the MPG figure you enter.
Questions people ask
Does my car emit more CO2 as it gets older?
Only through fuel economy. CO2 per gallon is fixed by chemistry, so an older car emits more per mile precisely to the extent that it burns more gallons per mile, and no more. Worn tires, dragging brakes, a failing thermostat that keeps the engine cold, a clogged air filter and degraded oxygen sensors all cost real fuel economy and therefore real CO2. What does change with age independently is the regulated pollutants — carbon monoxide, hydrocarbons, nitrogen oxides — because those depend on the catalyst and the fuelling control rather than on the carbon in the tank. A car can be emitting far more of those while its CO2 per mile is barely moved.
Why do you not include the emissions from making the fuel?
Because it would make the number harder to compare with the other figures people encounter, most of which are also tailpipe-only, including the CO2 on new-car labels. The upstream emissions are real: extracting, refining and transporting gasoline typically adds something in the region of 20 percent on top of combustion in published life-cycle accounts, and the equivalent upstream burden for electricity, methane leakage from gas supply among it, is also outside the grid emission factors most utilities publish. If you want a full life-cycle comparison, both sides need adjusting, not just the combustion side, and the result depends on which study you use. This page states its boundary and stays inside it.
How much difference does driving style really make?
It shows up entirely in the MPG you enter, and the effect is large enough to matter. Steady driving at moderate speeds against hard acceleration and high-speed cruising can differ by 20 to 30 percent in the same car, and short trips from cold are worse than that, because a cold engine runs rich and the first mile or two can consume several times the fuel of a warm mile. If you want to see the effect on this page, enter your winter average and your summer average separately and compare the two annual figures. The gap between them is the driving and the weather, expressed in kilograms.
Is 12,000 miles a year a lot?
It is a common enough figure to be used as a default in a great many places, which is why it appears here, but it says nothing about whether it is a lot for you. What matters for this calculation is only the product of miles and gallons per mile. Someone driving 6,000 miles in a 15 MPG truck emits about the same as someone driving 12,000 in a 30 MPG car, and the two people have very different options for changing it. Enter your own odometer difference over a year rather than a default, since it is the one input here you can measure exactly.