The end result? The US auto industry had to retool their factories and kill or redesign many of their products in order to meet the new standards (and many just paid the fines as long as they could). It was the largest change in the industry since WWII, and it destroyed almost all of their competitive advantage - outside of trucks which had relaxed standards - and resulted in decades of poor build quality that American cars are still disparaged for to this day. The Japanese and European automakers who operated mostly in markets that already demanded small cars weren't forced to retool and American car companies have been playing catchup ever since, without time to wind down their existing product lines and get the public used to small fuel efficient cars.
The fuel efficiency standards had to happen but the way in which they were implemented nearly brought down an entire industry, one that proved invaluable for national security during WWII when the allies needed to manufacture mountains of munitions and tanks. Governments can't implement huge policy changes without giving everyone time to adapt without causing massive unforeseen problems, ones which can only make the problem we're trying to solve a lot harder.