Auto makers don't have to use california's rules to determine how they design the whole US fleet. They choose to do so because the cost to comply nationwide is

Auto makers don't have to use california's rules to determine how they design the whole US fleet. They choose to do so because the cost to comply nationwide is low if they're already doing it anyway. A company could absolutely design cars for the 31 states with lax standards and attempt to compete with lower costs. That nobody has done so in the last 50 years seems to imply that you actually wouldn't see much in the way of savings. Realistically international car companies will want to make as few changes as possible between markets, and so even if California's regulations are removed we'll still have Europe effectively dictating the floor of emissions standards.