One would need to know how many km driven in a year, then convert the associated g to tonnes. In addition, one must be given more information, such as on average, there is 1.0 g CO2/km driven. Of course different vehicles would use a different amount of fuel, so that information would have to be considered if evaluating a specific vehicle. In conversion - Each g is one millionth of a tonne.
So 10000 km driven in a year assuming 1.0 g CO2/km would be 10000 g divided by 1000000 g/tonne or 0.01 tonnes CO2.
It is easy to be overwhelmed, especially if not given all the information. Being able To convert various units (eg, English, International, chemistry, physics, etc.) can become an important skill to learn in conducting research and reading technical papers.
In case you did not know, it was my guess for an example calculation of 1g/km. Dr Saxena gave the conversion reference to produce a more valid calculation, and could be adjusted based on fuel efficiency of vehicle, based on its petrol use efficiency. For research, it is important to reference the source, such as the sunearthtools.com references.
Porfavor, ¿Cómo convertir el factor de emisión (g / km) de CO2 a Toneladas/terajulios?