That was Obambi's fuel economy regulation. Trouble is, nobody will buy the resulting automobile. Heck you cannot get 54.5 MPG out of motorcycle, let alone any sort of decent car. Gasoline engine technology was well understood by the 1950's. We had cars that could do 20 mpg back then. Fifty years later the average car is not doing much better, maybe 24 mpg. No way will the technology get to 54.5 mpg for anything more than a motorcycle.
Especially as the Environmental Pollution Agency limits what they call "NOx" emissions. Air is 20% oxygen and 80% nitrogen. Heat air hot enough, say in an engine combustion chamber, and the oxygen and nitrogen go together forming various nitrogen oxides, nitrates, NO, NO2, N2O4, and a bunch more. Old style LA smog formed when the nitrates mixed with oily vapors and sunlight formed that yellow cloud we used to see over city skies. So, rather than clamp down on oily vapors from leaked fuel, the EPA decided to clamp down on NOx emissions. The only way you do that is lower the combustion temperate in the engine cylinders. Which ruins fuel economy and power. Car engines are heat engines, the hotter you can run them, the better they perform. If we dropped the NOx limit we would get a 10% maybe 20% performance improvement right then and there.
Or we could convert to battery powered cars. Trouble is, the best battery powered cars can barely get you to work and back. And they cost twice what a real car costs. No way could a battery car get you to a ski resort for a weekend, or even to grandmother's house unless grandmother lives right next door. The battery is the most expensive part of a battery car. Nobody dares say what battery life might be, but battery replacement is so expensive that you might as well scrap the whole car and buy another one, new. Plain old lead acid car batteries only last 4 winters. How long will your Tesla battery last?
So, Trump is telling Detroit that he is going to "review" (drop) the 54.5 MPG regulation which allows Detroit to keep making cars that people will buy. Good oh. Right on Donald.
Market pressure is there to get fuel economy up and keep it up, all things being equal, customers will buy the car with better fuel mileage and everyone in Detroit knows it.
And, don't fault people for buying SUV's You need an SUV to get Mom, Dad, the kids, and the luggage onboard. The little econo-boxes are horrible on a long trip with kids. Trust me on this.