Automakers, regulators, and environmentalists were sent scrambling to the negotiating table after President Barack Obama issued an executive order on Monday instructing the Environmental Protection Agency to reconsider California’s rules to regulate greenhouse-gas emissions from cars and trucks.

National laws require an average of 35 miles per gallon by 2020, but California’s regulations—considered by most to be tougher than federal rules—could require the rough equivalent of more than 40 mpg.

Yesterday, the nonprofit Aspen Institute said it had convened representatives from Ford, General Motors, Honda, Toyota, the Sierra Club, the Union of Concerned Scientists and the Natural Resources Defense Council to discuss the matter. According to the Institute, the parties “explored ways to achieve national and state goals for reduction of greenhouse gas emissions and fuel economy improvements.”

The New York Times reported that another private meeting will be held next week in Los Angeles with midlevel auto executives, state regulators and environmental leaders trying to work out a compromise. Perhaps for the first time, all the major players are showing real interest in fuel efficiency and tailpipe emissions rules that satisfy targets at both the national and state level.

A Battle Over Numbers, and More

Automakers will argue that federal targets—signed by former President Bush in 2007 and set to take effect with 2011 vehicles—are more stringent than California’s objectives.

Charles Territo, spokesperson for the Alliance of Automobile Manufacturers, an association of 11 vehicle manufacturers, told HybridCars.com, “Applying the California standards to the rest of the country will actually produce lower fleet-wide fuel efficiency standards than the federal standards as proposed under the Energy Independence and Security Act.”

In fact, a comparison study conducted by the California Air Resources Board (CARB) in February 2008 shows that in 2015, for example, California rules would produce a fleet fuel efficiency average in many states, although not in California, of 31.3 mpg—while federal guidelines would yield 31.8 miles per gallon. In another example, the fuel efficiency of trucks in 2015 in California would be 25.5 mpg under California rules, but 27.1 mpg under the federal law. (Fuel efficiency varies state-by-state depending on the number of cars versus trucks and large SUVs in those states.)

“I can see how they can torture the numbers,” said John DeCicco, senior analyst at the Environmental Defense Fund, in an interview with HybridCars.com. “The bottom line is that the California standards are far more effective in reducing emissions.” That point was also made in the executive summary of the CARB study: “California’s rules provide superior greenhouse gas benefits.”

Disagreements over competing numbers in forecasts and studies are unlikely to be resolved in upcoming meetings—because the underlying fight is for the power to establish the rules, and the ability of California to apply pressure for reform on urgent environmental issues. “Don’t be confused by pointing out selective numbers,” said DeCicco. “There is 40 years of history of California leading and the feds catching up. In each case, the automakers blocked progress. We are concerned if that ability to lead is taken away, what will we be left with?”

With the threat of California rules looming—and the fate of Detroit carmakers hanging on continued federal bailout money—automakers appear more prepared than ever to agree to aggressive fuel efficiency improvements that can work for all 50 states.