Cheers Peter, I was just going by what I know and understand. That being the more efficient the car runs the higher the Co2% output. Then the output is again inceased by the operation of the catalyst. The Co2 output is considerably higher today than 10 years ago in percentage terms (about 15% in my rough calculation).
I usually learn something from your posts and this time is no exeption. You have now enlightened me that it works out less in terms of Grammes per Km.
Don't bust a gut looking for it but that info you have would be interesting if you come across it sometime.