Quote:
"The emission standards for trucks (lorries) and buses are defined by engine energy output in g/kWh; this is unlike the emission standards for passenger cars and light commercial vehicles, which are defined by vehicle driving distance in g/km - a general comparison to passenger cars is therefore not possible, as the kWh/km factor depends (among other) on the specific vehicle".
Source:
https://en.wikipedia.org/wiki/European_emission_standards
So, as your vehicle is a car, the figure on your V5C should be in g/km.
I've been scratching my head trying to work out where the apparent anomaly in your figures could come from. Looking at the V5C for my own vehicle though suggests that you might be mixing up carbon monoxide (CO) with carbon dioxide (CO2) on the form.
Under 'Vehicle details', on the left hand side of the two-page spread, there is indeed a section headed 'Exhaust Emissions', with CO having '(g/km or g/kWh)' alongside it. However that's the figure for carbon monoxide, not carbon dioxide. If you look higher up on that side of the page, next to 'V.7', you'll see the figure for carbon dioxide emissions.