In looking to fill up wife's new Ford Focus, we used the premium 93 stuff.... but then did some web surfing. It appeaes that the octane rating does not improve mpg, but is merely aimed at timing and engine knock.
I could swear I got better mpg in our 2001 Jeep GC with 93 or 89.. but maybe just an illusion or wishful thinking. Ok... I am going to only use 87 in our son's car, and see how things go.
---- he said ---- Most engines have a knock sensor and spark retard.
More spark retard means more compression and more power. If the car senses a knock then it will back off the spark retard.
So in theory, they all provide the same power at the same compression. Really only matters in tubo/supercharged cars or with really high compression piston (like my Ninja sportbike)