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Emissions from cars

Summary

The calculator uses a factor of 14.3 kg CO2 per gallon of fuel.
Carbon dioxide arises from
  • the burning of fuel
  • fossil fuel energy used in the extraction of oil
  • energy used in oil refining
  • construction of cars and their maintenance
  • construction of roads and their maintenance

Details

CO2 from fuel use:
The CO2 emissions from petrol and diesel are around 2.4 kg per litre [1], i.e. 10.9 kg per gallon (since 1 gallon is 4.546 litres).

Energy used in oil extraction:
The CO2 generated per person in oil extraction together with gas extraction is estimated as 0.28 tonnes per year (from source [2], chart A). Since oil use and gas use are roughly equal in the UK in energy terms, we can allocate half (0.14 tonnes per year) to each of oil and gas.

Energy used in oil refining:
This is estimated as 0.43 tonnes per person per year (source [2], Chart A, "Refining industries").

Construction of cars and their maintenance:
From a financial point of view, fuel costs are just a small part of car ownership - the RAC gives the cost per mile for a car up to 1500cc to be 37.3p [3], but only 25% of this is fuel. The other costs are not as energy-intensive as fuel, but manufacture and maintenance clearly involve fossil fuel use.

According to source [2] (Chart D), 26.0 million tonnes CO2 per year (7.10 million tonnes Carbon) arises from fossil fuel use in "Motor vehicle production". This is equivalent to 0.43 tonnes CO2 per person, and this is the estimate used here to derive the factor used.

A second estimate (not used here) is that 0.099 kg/km CO2 arises from UK car maintenance and manufacture, quoted in source [4]. This is equivalent to 0.158 kg/mile, or a total of 38 million tonnes CO2 for the 27 million cars in the UK travelling an average of 9000 miles per year, i.e. a considerably higher estimate.

Construction of roads and their maintenance:
An uncertain amount.

Derivation of the factor used

(Figures are per person per year in the UK where not otherwise specified).

Using the following data:
  • oil use in the UK (excluding non-energy use) is 78.1 million tonnes [5], i.e. 1.29 tonnes per person
  • the energy content in crude oil is 11.63 MWh / tonne [6], and
  • CO2 emissions from petrol, diesel and aviation fuel are around 0.245 tonnes / MWh [1],
calculation gives the CO2 emissions from using oil as energy to be 3.67 tonnes per person. Almost all of this is used in internal combustion engines.

Adding the 0.14 tonnes CO2 from oil extraction and the 0.43 tonnes lost in oil refining gives a total energy wasted in oil exploration and refining of 0.57 tonnes, which means that only 3.10 tonnes of the total 3.67 tonnes is actually used in internal combustion engines (an efficiency of 85%).

Allowing for the 0.43 tonnes CO2 arising from fossil fuel used in motor vehicle production, gives a total of 4.10 tonnes per person used in transport, of which only 3.10 tonnes is actually used in internal combustion engines.

CO2 emission rates therefore need to be scaled up by an addition of 32%.

Emissions of 10.9 kg per gallon should therfore be scaled up to 14.3 kg CO2 per gallon of fuel.

Fuel use data have been taken from the Which? magazine Car Guide [7]. These are measured values under real world conditions rather than the manufacturer's published figures under ideal conditions. The published values have been grouped and averaged as follows:
SizeAverage mpggCO2 per mile*
Small, medium, large, city and estate cars52280
MPVs and small & medium SUVs46310
Sports cars and large SUVs35410
*calculated from mpg and CO2 emissions of 14.3 kg per gallon


Miscellaneous points

  • The RACís vehicle running costs tables for new cars [3] give estimates of the miles per gallon for different engine sizes.
  • The number of private cars in the UK is 27 million, i.e. just under 1.1 cars per household [8]
  • The proportion of households without a car is 26% [8]
  • 59% of road transport emissions is from cars (12% from light vans, 28% from trucks and buses) [9]
  • 88% of car CO2 emissions is commuting and other private mileage (Defra, [10]), rather than business.
  • Average car mileage is 9000 per year [8]

References

[1]DEFRA (2007) Guidelines to Defra's GHG conversion factors for company reporting www.defra.gov.uk or www.carbonindependent.org/files/conversion-factors.pdf
[2]Carbon Trust (2006) The carbon emissions generated in all that we consume www.carbontrust.co.uk or www.carbonindependent.org/files/ctc603.pdf
[3]RAC Motoring Services (April 2007) Vehicle Running Costs www.emmerson-hill.co.uk or www.carbonindependent.org/files/motoring_costs_2007.pdf
[4]John Barrett, Anthony Scott and Harry Vallack (2001) The Ecological Footprint of Passenger Transport in Merseyside www.merseytravel.gov.uk or www.carbonindependent.org/files/ef_of_passenger_transport.pdf
[5]BERR Energy Trends December 2007 www.berr.gov.uk/files/file43304.pdf or www.carbonindependent.org/files/file43304.pdf
[6]Carbon Trust (2006) Energy and carbon conversions (CTL004) www.carbontrust.co.uk/publications/publicationdetail?productid=CTL004
[7]Which magazine (2019) Car Guide 2019/20 https://www.which.co.uk/
[8]Chris Goodall (2007) How to live a low-carbon life (Earthscan)
[9]AEA Technology (2005) Environmental Costs of Rail Transport www.rail-reg.gov.uk or www.carbonindependent.org/files/aea_enviro_rep.pdf
[10]DEFRA (2007) Act on CO2 Calculator: Public Trial Version Data, Methodology and Assumptions Paper www.defra.gov.uk or www.carbonindependent.org/files/actonco2-calc-methodology.pdf


First published: 2008
Last updated: 29 Apr 2021