The Environmental Protection Agency (EPA) is holding a hearing today on the waiver request by California, as well as more than a dozen other states, to allow higher auto fuel-efficiency standards under the Clean Air Act.

The Detroit Three -- General Motors, Ford, and Chrysler -- are not sending direct spokesmen to the event. But one of their home-state senators, Carl Levin (MI), is there, and his argument tracks with what the auto industry wants: a "single national standard" to govern auto tailpipe emissions.

That doesn't sound so bad, does it? Well, the Clean Air Act did allow California to set its own environmental regulation standards and give other states the authority to opt in, but let's assume that a national standard would be the best solution for automakers as well as the nation. Now where should the national standard be set?

When I asked Levin this question last week, he said any national standard should simply be "fairly achieved" and that the specific fuel-efficiency level should be "left to the experts."

But Dan Becker, director of the Safe Climate Campaign and the Sierra Club's former senior global warming advocate, sees the "national standard" push in a different way: as Detroit's code for urging rejection of the California waiver. "When they say 'one national standard,'" Becker told me, "what they mean is ... [that] California['s waiver] should be obliterated and the EPA should keep its nose out. That ain't gonna happen."

What may happen? The automakers may end up ruing the day they advocated for a uniform national standard.

"What looks as if it might happen is the federal agencies ... may eventually meet California where California is," Becker added. "In other words, they'd set a national standard for auto fuel economy and emissions at the California level."

If you think the automakers would be okay with a national standard at California's level, check out the National Automobile Dealers Association's recent report blasting the state's stronger emissions rules. The auto dealers, reflecting the industry's overall view, appear to assume that a "national fuel standard" would be lower than California's level.

Particularly notable is the automakers' alarm at the idea of eliminating "mix shifting," which allows them to sell smaller numbers of cars with poorer fuel-efficiency records as long as the overall average of cars sold complies with the cleaner standard. Here's the passage of note:

Absent mix shifting, the only way to comply with [California]'s fuel economy/GHG regulation is to deliver for sale in each [California-backing] state a new vehicle fleet that, on average, emits significantly less CO2, which can only be achieved by significantly improving fuel economy.

Significantly improving fuel economy?! As awful as it sounds to Detroit, that may just be the future.