2kg of CO2? Atomic weight of CO2 is about 44, of which carbon is 12, so 27% of CO2 is the carbon from the gasoline. I know that gasoline contains more than just hydrocarbon chains, and that the chains also contain hydrogen. But for the sake of this back of the envelope calculation I'll disregard both.

27% of 2kg is 0.54kg, according to https://en.wikipedia.org/wiki/Gasoline a liter of gasoline is 0.755kg. Aka 2kg of CO2 is the result of burning 0.72L of gasoline. Driving 4miles, or 6.44km on 0.72L is 9km/L, or 21.2mpg. 1.6kg of CO2 would be 0.57L and 11.3km/L or 26.6mpg.

Maybe I shouldn't have disregarded the additives and the hydrogen, but unless they account for about 50% of the weight of the gasoline, then those 4 miles were driven in a something very uneconomic.