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The figure most UK calculations use is about 0.164 kg CO₂e per kilometre for an average petrol car, which is ≈ 0.264 kg per mile. Diesel is broadly similar; a larger or older car is more, a small efficient one less. Over 1,000 miles that's about 0.26 t — roughly 13 trees' worth of a year's absorption.
The number that matters is the one for your car and mileage — TreeMonkey lets you slide it and see the total in tonnes, trees, flights and homes, each with its source.
Source: DESNZ/DEFRA GHG conversion factors — average car. Figures are illustrative UK equivalents.
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Frequently asked questions
Petrol vs diesel per mile?
They come out close on a per-mile basis for an average car; the bigger differences are engine size, age and how you drive. TreeMonkey uses an average-petrol-car factor for illustration.