There's a reason why auto manufacturers somewhat surprisingly stood behind President Obama's announcement this week about substantially raising the mandated corporate average fleet economy (CAFE) standard. The change to a 39 miles per gallon average for passenger vehicles by 2016 won't be as much of a burden on them as it appears.

According to automotive publisher Edmunds.com, the CAFE standard is computed differently than the EPA's MPG estimates that appear on window stickers, so the ratings on vehicles for sale will be noticeably lower.

The disparity in the CAFE calculation versus the real world isn't new, it's just more noticeable now. The government's CAFE requirements are based on tests on a dynamometer machine that simulate road conditions. The EPA downgrades those results in its estimates because they were found to not be reflective of the real world. However the CAFE numbers used to benchmark auto makersremained the same.

Here's how the National Highway Transportation Safety Administration explains it on its website:

"Three different sets of fuel economy values- NHTSA’s CAFE values, EPA’s unadjusted dynamometer values, and EPA’s adjusted on-road values exist. NHTSA’s CAFE values are used to determine manufacturers’ compliance with the applicable average fuel economy standards and to develop its annual report, the Automotive Fuel Economy Program Annual Update. The EPA’s unadjusted dynamometer values are calculated from the emissions generated during the testing using a carbon balance equation... EPA’s adjusted on-road values are those values listed in the Fuel Economy Guide and on new vehicle labels, adjusted to account for the in-use shortfall of EPA dynamometer test values."

The test simulation MPG estimates have been well documented to stray from reality, for years overstating the performance of hybrid vehicles before being corrected in 2007.

“Turns out that there are loopholes almost big enough to drive an SUV through,” said Edmunds.com CEO Jeremy Anwy.

According to Edmunds.com, 29 car models and 36 truck models already achieve the new standard, and about a third of the cars and half of the trucks are produced by a domestic automaker.

The new CAFE regulation of a fleet-wide 35.5 mpg by 2016 will still be a steep incline in mpg for auto makers who are barely meeting today's standard of 27.5 mpg for cars and 24 mpg for light trucks.