Summary

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

Detail

• CO2 from fuel use: the CO2 emission from petrol and diesel is around 2.4 kg per litre [18], 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 0.28 tonnes [from source 7, chart A]. Since oil and gas use is similar in the UK (in energy terms), we can allocated half (0.14 tonnes) to each of oil and gas.

• energy used in oil refining: This is 0.43 tonnes per person per year [source 7, 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 [20] gives the cost per mile for a car up to 1500cc to be 37.3p, 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 [7] (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 is that 0.099 kg/km CO2 arises from UK car maintenance and manufacture, quoted in source [22]. 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 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 [36], i.e. 1.29 tonnes per person
• the energy content in crude oil is 11.63 MWh / tonne [25], and
• CO2 emissions from petrol, diesel and aviation fuel are around 0.245 tonnes / MWh [18],
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 is 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.

Vehicle emission data is available for new car registrations, but this is measured under ideal conditions and it is necessary to add an amount to allow for 'real world' effects on fuel consumption (driving style, passengers, luggage, poor maintenance, weather, under-inflated tyres, use of air-conditioning etc). These have been estimated to increase fuel consumption by an average of 15% [Defra, 14].

Taking weighted averages of DEFRA car emissions factors [19] which allow for these 'real world' effects gives

Size	Engine size (litres)	mpg	gCO2 per mile*
small car	< 1.5	37	390
medium car	1.5 - 2.0	33	430
large car	> 2.0	24	600

* calculated from mpg and CO2 emissions of 14.3 kg per gallon

Miscellaneous

The RACs vehicle running costs tables for new cars [20] gives estimations on 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 [10]

• the proportion of households without a car is 26% [10]

• 59% of road transport emissions is from cars (12% from light vans, 28% from trucks and buses) [23]

• 88% of car CO2 emissions is commuting and other private mileage [Defra, 14], rather than business.

• average car mileage is 9000 per year [10]

References

See Reference List