The mandated CAFE rules coming in 2015 for North American new cars will have one manufacturers' products meet stricter fuel efficiency numbers than others. The standards will not just be measured by the vehicle's footprint, but also how many are made by the manufacturer. For example, larger trucks from manufacturers such as GM's will need to be less fuel efficient than Suzuki's, by about 6mpg according to NHTSA estimates for the 2015 model year.

The footprint is calculated in squared feet as the relative area between a vehicle's tires. As such, the proposed federal fuel economy regulations will see the highest relative fuel efficiency increase in the smallest vehicles.

Furthermore, the CAFE law will see a 25 percent improvement over today's average fleet consumption in the five-year period from 2011-15, with a 40 percent improvement by 2020, where the industry average is to be 35mpg. The National Highway Traffic Safety Administration developed one set of rules for cars and another for trucks. Each automaker's goal for either category is expressed in mpg and based on how many vehicles of each footprint it sells.

Porsche would be most affected by the law, as its relatively short-wheelbase cars would need to average 41.5mpg by 2015 as opposed to today's requirement of 27.5mpg: a near 51 percent increase.

Makers such as Porsche may opt to pay fines instead of trying to meet such lofty goals, and risking diluting the brand and making it unattractive to customers, not to mention the high costs involved in doing so.

The NHTSA is open to public comments regarding this latest proposal for the next two months, as final rules must be adopted by April 1, 2009, while the Bush administration plans to act by year's end.