Robinson Meyer:

When the automakers endorsed Obama’s cafe standards, they still exacted two concessions. These set the stage for what was to come.
 First, the new cafe standards would apply differently to different cars. Light trucks would have to meet less stringent rules than cars. And all the rules would automatically adjust to match the “footprint” of new cars—the idea being that the rules should account for the size of car that’s popular with consumers. If one automaker sells mostly crossovers and pickups, it shouldn’t be held to the same standard as another that sells mostly sedans and coupes.
 Second, the rules would be revisited. The EPA and the Department of Transportation had to look at the data anew and tweak the rules to match reality, and they had to do it “no later than April 1, 2018.” Since the toughest rules were always scheduled to bite in the late 2010s, the car industry would basically get one more chance to fight.
 Four years in advance of that date, the EPA began making sure it would meet the deadline. Agency researchers spent two years poring over thousands of pages of economics, engineering, and air-quality research. In July 2016, they published their preliminary conclusions. In a 1,200-page report, the EPA, the DOT, and California argued that new technologies would let carmakers hit their goals even more cheaply than once anticipated. The rules would also improve safety and add jobs, they said.
 In short: The cafe standards were working, and they should not change.