No difference unless your engine is designed to use high octane fuel. Putting premium in the tank of a car that uses regular is just a waste of money.
Same thing. If you're lucky, and the crap in the bottle burns as well as gasoline, you get a little more
mileage that way.
The solution is to drive smaller cars with smaller engines. I used to think that the "Mad Max" movies were silly with people driving big cars during a severe fuel shortage. North America's love of the SUV has changed my mind.