Of course we are, and have been doing for ages. The difference is that the trash cars end up being replaced by giant comic book spaceship cars that use the gain

Of course we are, and have been doing for ages. The difference is that the trash cars end up being replaced by giant comic book spaceship cars that use the gains in fuel efficiency to add more bulk to the vehicle and still emit as much nasty. Switching to replacing the existing dirty new vehicles with electric transport won't entirely remove the problem, but it will at least stop making it worse.