About time, the automakers were using the CAFE standards as a pretext to cover moving production outside of USA. Both the republicans and the democrats obliged. Trump doesnt really care one way or The other about about fuel economy, but he does care about keeping factories in USA. He is calling the auto makers bluff.
Remember, the automakers are very skilled at zero sum negotiation. They really dont like this. If the automakers dont commit to USA production, trump might just extend california's regs to cover the entire nation. Quid pro quo.