There is a principle of constitutional law that states may not impose laws that burden the commerce of other states. This rule can be relaxed, however, if the federal government permits the states to impose burdens on the other states. Since the manufacturing of cars has a national market, it makes sense that, if there are to be any regulations on the industry, it ought to be a national standard as it is hard to comply with 50 different state jurisdictions. However, President Obama wants to turn this rule completely upside down, and enable California's government to impose a fuel efficiency standard on the sale of cars that exceeds the federal standard.
This would create a large number of problems for an industry that is already nearly bankrupt. Not only would the car industry have to sell cars that are specifically made for the California market, it would likely end up selling the same cars to other markets as well. As a result, California's burdensome rules will have the effect of regulating the sale of cars outside the state, which the negative commerce clause states should not occur. Thus, those of you who live in Nevada: you are likely to drive the same over-regulated clunker cars that Californians will be forced to buy, even though you don't live in California.
Further, fuel effeciency standards have killed about 2,000 Americans a year on the highways. Why? Well, smaller and lighter cars are less crashworthy, and thus, your chances of survival are lower in these cars. Further, if consumers truly wanted a car with more mileage, there are plenty of cars available to purchase. However, the government realizes that there isn't a demand for those types of cars, because they cost more, are less safe, and are less fun to drive. Thus, Obama wants to force you to drive a car that you otherwise would not drive. This is an issue of freedom, and it is slowly drifting away.
Welcome to Obama's over regulated America.