How it is worked out
A car's CO₂ is the fuel it burns: km ÷ mileage × 2.31 kg per litre of petrol, shared by everyone in the car. Buses, trains and planes use CO₂ per person per km, because they carry many people.
The starting factors are typical for India: buses about 0.03 kg per person-km when reasonably full, Indian Railways under 0.01 because most trains are electric and full, and economy flights about 0.15 for the CO₂ alone.
Flights do extra harm high up: contrails and nitrogen oxides add warming on top of the CO₂. Official UK factors multiply flight CO₂ by about 1.7 to include it.
Frequently asked questions
Is a full car better than a train?
Four people in an efficient car can come close to a bus per person, but a train is usually still lower, especially an electric one.
Why is a short flight so bad per km?
Take-off and climbing burn the most fuel, so short flights use more fuel per km than long ones.
What about an electric car?
Use the petrol vs EV calculator: on India's grid an EV puts out about 0.1 kg per km for a car, less with rooftop solar.
Sources
- UK government GHG conversion factors ↗ – flight, rail and bus factors, and the high-altitude factor
- India GHG Program ↗ – Indian road and rail emission factors