If this pace continues through 2020, it would exceed the effective 42mpg that would be required by the proposed California CO 2 limits. Since the federal rules would exceed California's requirement, that raises the obvious question of whether that means the automakers would stop fighting the California rules. According to Martin, the issue with the state rules is not so much the mileage requirement. The real problem is the "17 or 18 distinct compliance plans that an automaker would have to manage" due to the patchwork of rules. Not all the states are taking the California rules in whole, some are cherry-picking and paperwork has to filed in each state verifying that the manufacturer is in compliance. This can be particularly problematic if the rules are based on sales within a given state and sales of different types of vehicles vary from state to state. A single national rule simplifies things and allows manufacturers to aggregate sales averages over the whole country.
[Source: General Motors]