It is true that as an abstract economic principle, the case for tightening fuel-efficiency standards in American cars will remain strong even if oil prices remain low.
But this is a case where politics matters more. The rules are up for review in 2017. President Clinton will maintain them. President Rubio will not, although he may not weaken them by as much as people think for the simple reason that the automakers prefer national rules than the segmented market that would result if the feds loosened the 54.5 mpg-by-2025 target.
The basic problem is that California can set its own air-quality rule, which are then automatically followed by Connecticut, Maine, Maryland, Massachusetts, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington. It does not set fuel-efficiency standards per se, but it can reject federal rules that are not consistent with its goals. If the feds loosen the 2025 standards, then California is likely to tighten its air-quality regulations in a way that effectively raises the fuel efficiency standards.
Auto companies will not like that. The upshot is that President Rubio would likely loosen the goals if oil prices remain low, but not abolish them altogether.
Anyway, Saudi Arabia still seems to be aiming for low oil prices. Whether that is a sensible bet remains to be seen.