As reported by Scientific American:
Trump’s draft plan would have manufacturers selling cars to reach an average target of 40 mpg by 2025. California’s rules still require an average of 54.5 mpg by then. If the country splits into two auto markets, experts say, manufacturers will likely meet the requirements by manipulating prices in the near term, because technological changes take more time to develop.
“There’s going to be some shuffling,” John DeCicco, a research professor at the University of Michigan Energy Institute, said recently. “If they have to sell a more efficient mix of cars in California, they’ll have to price the more fuel-consumptive cars higher, to suppress their sales. They’ll have to put more incentives, discounts, dollars on the hood for more fuel-efficient vehicles, to press those, compared to Ohio and Nebraska”