That's the question that Michelin executive Patrick Ozoux wants the EU to answer. According to a story in Automotive News Europe, Ozoux said that, "When carmakers fit such tires as original equipment, they should get a bonus on their CO 2 rating." The EU will likely be assigning a CO 2 rating to new cars in the coming years.
I suppose the important consideration for the regulators is how long does the CO 2 reduction last. Is it for the life of the car? Probably not, since the tires will likely be replaced at some point. Then again, I'm sure the CO 2 ratings would naturally be based on the car as sold, right? In that case, it only makes sense to accurately assign a rating that includes the reduction from the low rolling resistance tires.