This is the summary page for the car transport tag, but there are also lists of other car transport wikipages, as well as of forum posts and files.
Any car transport footprint depends not only on the mileage travelled, but also on the fuel source (the biofuel vs. fossil fuel mix) and the efficiency with which it is burnt by the car (the combustion efficiency).
Fuel life-cycle footprint
The carbon emissions associated with any fuel are not limited to those directly put out in exhaust fumes. They also include emissions resulting from processing and transport, and – for biofuels – those from land conversion, fertilisers and harvesting.
Combustion efficiency
Fuel combustion efficiencies depend on the make and model of the car (i.e. its engine, weight, aerodynamics etc.), as well as how the car is driven (i.e. speed, gear changes). Models have been given average emissions ratings per mile travelled (in gCO2/km), which are available from http://www.vcacarfueldata.org.uk/search/search.asp. Note, however, that these figures can vary with driving style and whether driving is predominantly in urban or rural areas.
Measuring car footprints
Car transport footprints can be calculated from your mileage and an average emissions conversion factor for your make and model of car (in gCO2/km); or directly from the volume of fuel you consume and its carbon content per unit volume (in gCO2/l). The former benefits from being easily recordable and verifiable (based on your annual MOT); whilst the latter is more difficult to record, but is more accurate because the volume of fuel combusted (and hence the carbon emissions) are measured directly.