"I was just about to respond that the Co2 after the cat was more than before the cat. So this proves new cars are producing more Co2. But after reading your post again I assume you mean that less C02 is produced during the manufacturing process of the car itself."
Percentage wise it looks bad - heh???
BUT, exhaust gases are measured in grams per vehicle km for evaluation and legaslative purposes and CO2 is much lower per total exhaust output than it was 10 years ago. Basically engines of similar size are more efficient than they were 10 years ago so they use less fuel and therefore emit less emissions re grams per vehicle km and this includes CO2.
If you want some figures, I'll "dig them up". I did a research report on OZ vehicles from 1972 to 1986 re emissions and grams per vehicle km (which is the standard all our vehilces are tested against to meet Australian Design Rules - ADRs)