Emissions from the combustion of different fuels vs. emissions from different mobile source categories.
Mobile source CO2 is formed by burning fossil fuels. Virtually all of the carbon in the fuel is converted to CO2. Therefore, and considering that CO2 remains in the atmosphere for a long time, national aggregate consumption of different types of fuels provides the most accurate basis for estimating CO2 emissions.
IPCC guidelines for national reporting of GHG emissions account for this fact, and EIA and EPA both use fuel consumption—not vehicle sales and fuel economy—as a basis for estimating and reporting CO2 emissions. According to the IPCC (emphasis added), “Emissions of CO2 are best calculated on the basis of the amount and type of fuel combusted (taken to be equal to the fuel sold, see section 22.214.171.124) and its carbon content. ,,