Hmm... I've changed my mind, too. This is a bad idea.
It's true there will only be two CO2/mpg standards - CARB and federal (CAFE). But whereas CAFE requirements reflect the nation's fleet of vehicles, each individual CARB state would have to meet the required MPG average independently.
So even between two CARB states that both require, say, 31.3 mpg, manufacturers would have to adjust their fleets differently for each state. To comply with CAFE, manufacturers currently manage their sales mix through pricing (in order to reduce demand, and thus, sales, of certain vehicles, they raise prices) or rationing. Imposing fuel economy regulations on an individual state level will force manufacturers to have different MSRPs for each state, or more likely, ration the types of vehicles for sale in each state. This would be tough on small CARB states with few dealerships - DC's Volvo dealership would cease to exist unless they started selling Fords as well.
In essence, manufacturers would have to comply with an additional 17+ new jurisdictions, regardless if their standards are the same. All this red tape would be an inefficient use of resources and accomplish very little.
So I've got a genius idea: Instead of being individual states that happen to share the same CO2/mpg standards, why don't the CARB states combine all their vehicle sales into one pool, and have that pool of vehicles meet CARB standards?
Then we'll truly have two systems, one country - not seventeen systems, one country. Instead of having statewide averages, we will have a CARB-wide average.