Between now and 2025 the EPA is conducting studies on where fuel economy technology stands at the moment and getting public comment. A preliminary report on features like direct injection engines, start-stop systems, and small-displacement turbocharged motors is going to come out in June 2016. As part of that, the EPA will also look at customer response to the technologies, Grundler says that, at the moment, buyers are in favor of most of them, save for stop-start and CVTs. An important part of the consumer equation will be what people are actually buying, so if the small crossover boom continues for years hence, Grundler said, "The standards adjust with sales mix. We are not forcing everyone into small cars."
After the 2016 report, the EPA will gather public comments. Then, taking those into account, the final decision will come around April 2018, and it is still wide open: the proposed target could remain the same, or go up or down. "The EPA administrator makes the final decision," Grundler said, "and he will work for the next president."
He also said that he expects highly-efficient gasoline engines to remain at the forefront of fuel economy gains. Right now, if we use 54.5-mpg number as the guide, the EPA says automakers are ahead of schedule, and UK automaker consulting firm Ricardo said the standard is "attainable using known technology and minor electrification." So in spite of the occasional grumbling, it's probably best to think of the proposal as the real deal for now.