Where car emissions come from
Burning fuel turns the carbon in it into carbon dioxide. Because the carbon content of fuel is almost fixed, the CO₂ a car emits is directly proportional to the fuel it burns.
- Gasoline: about 8,887 g of CO₂ per gallon (EPA)
- Diesel: about 10,180 g of CO₂ per gallon
- Electric: energy used × emission factor of the grid that supplies it
The calculator divides those figures by your fuel economy to get CO₂ per mile, then multiplies by the miles you drive each year.
Worked example
A gasoline car averages 25 mpg and is driven 12,000 miles a year.
- CO₂ per mile: 8,887 ÷ 25 = 355 g.
- CO₂ per year: 355 g × 12,000 = 4.27 metric tons.
- CO₂ per month: about 355 kg.
With two people in the car, the CO₂ per passenger-mile falls to 178 g, which is why carpooling is one of the quickest ways to cut transport emissions.
Gas, diesel and electric compared
| Vehicle | Consumption | CO₂ per mile |
|---|---|---|
| Gasoline car | 25 mpg | 355 g |
| Gasoline car | 40 mpg | 222 g |
| Diesel car | 35 mpg | 291 g |
| Electric car, US average grid (0.37 kg/kWh) | 30 kWh/100 mi | 111 g |
| Electric car, low-carbon grid (0.10 kg/kWh) | 30 kWh/100 mi | 30 g |
The grid factor matters a lot for EVs. States that rely on hydro, nuclear or wind power have much cleaner electricity than states that rely on coal. The EPA’s Power Profiler gives the factor for your ZIP code.
What this estimate leaves out
The calculator counts the CO₂ that comes out of the tailpipe, or from the power plant for an EV. It does not include:
- Fuel production: drilling, refining and transport add roughly 20 to 25% to gasoline’s tailpipe emissions.
- Vehicle manufacturing: building a car emits several tons of CO₂, and more for an EV battery.
- Other greenhouse gases from air conditioning refrigerants.
Life-cycle studies still find that EVs emit less over their lifetime on the US grid, because the extra manufacturing emissions are usually repaid within one to two years of driving.
Ways to cut your car’s footprint
Drive smoothly, keep your tires inflated, avoid idling, combine errands, remove unused roof racks and stay near the speed limit on highways. When you replace the car, fuel economy or an electric drivetrain makes the largest difference.