The difficulty facing the U.S. automotive industry with the recently-revised 31.6mpg by 2015 CAFE requirement is well-known, but no less pressing. The carmakers claim the costs to comply will be measured in the billions of dollars when they can least afford it, and now a draft NHTSA report on the environmental impact of the fuel economy improvement has stirred further animosity toward the agency.

According to the draft report, the proposed fuel economy increases would result in a global reduction of atmospheric CO2 by 0.2ppm by 2030 and prevent the emission of over 400,000 tons of CO2 each year by 2035.

Those conclusions, and others, have caused the Alliance of Automobile Manufacturers to take issue with the assessment, calling it alternately 'illogical' and 'wholly inconsistent', reports The Detroit News.

The Alliance believes the NHTSA's model is dramatically flawed, and seriously overestimates the impact the regulations could have on the global atmosphere.

The report itself is the product of a court-ordered investigation into the issue of the environmental impact of fuel economy regulations. Now the NHTSA is refusing to comment, since the report is only a draft, but the Alliance is speaking freely. Their primary point of contention with the NHTSA finding that the heightened fuel economy standars will reduce CO2 emissions lies in the difference between theory and real-world market effects.

By raising fuel economy standards, the automakers argue, the government is effectively raising car prices, which will cause consumers to hold onto older, more polluting vehicles, longer. That, in turn, will counter-act any savings from newer cars.