The simplest way to calculate tailpipe emissions of CO2 is to measure fuel consumption. Just add your fuel bills each period to see how much CO2 you have produced.
Calculate CO2 by amount of fuel used:
Calculate CO2 by amount of money spent on fuel:
Calculate CO2 by mileage:
Burning a litre of diesel produces around 2.62 kgs of carbon dioxide, whereas petrol has a lower carbon content and produces about 2.39 kgs. Older engines might lose a few percent due to unburnt fuel, but otherwise technology can have little effect on this chemistry.
The table found here shows some variation on current vehicles, but since the figures are a composite of separate tests it is quite likely that inaccuracies will creep in.