Though opposed by most environmental groups, this interim evaluation, according to Automotive News (sub. req.), will likely be conducted between 2019 and 2021. Automakers argue that their ability to hit the lofty target of 56.2 miles per gallon proposed by the Obama administration hinges on unpredictable future sales of electrified vehicles like the Nissan Leaf, Chevrolet Volt and other upcoming plug-ins. Hence the need for a midterm review. Basically, if sales of plug-ins take off, the proposed 56.2-mpg target will not be altered. On the other hand, if plug-ins tank, expect the 56.2 mark to be adjusted downwards.
[Source: Automotive News – sub. req.]