Maybe. However, I think the car company approach is going to be to argue to the courts that only the feds have the power to mandate fuel economy standards. Personally, I think they should have financed an initiative so that the voters of California could have slapped this idiotic law down. And they would have.
The feds have enacted legislation that expressly gives California the power to enact stricter standards than those that apply to the rest of hte country. This dates back to the early 70s and the first version of the Clean Air Act.