Doesn't the octane rating of premium vary from state to state? Could be 90, 91, 93 or even 94? Furthermore how does one know the "amount of additives" found in Premium? Or, from one brand to another or one refinery to another? Many unanswered variables that N=1 would have trouble answering.
But, Chevron states FE is proportional to heating content of the gas not octane as modern engines adjust timing via use of knock sensors. They also state that premium fuel may have a 1% higher heating value than regular gas.
With respect to additive, there are many additives: antiknock, demulsifer, inhibitors for oxidation and corrosion and so forth. In addition there are deposit control additives with a minimum amount required to be added by regulation. Top Tier gas suppliers have agreed to increase the amount of deposit control additives above Fed. requirements. One might, therefore, expect better performance (FE) from cars using more deposit control additives than less as there should be less chance of deposits in engines using Top Tier gas. Regular and premium gas from Top Tier suppliers have the same amount of additives. I only use Top Tier gas.
Lastly, ethanol is added to virtually all gas to meet EPA emissions requiements. The amount as reported to the EPA has been near 10% for many years as the EPA emissions requirements ramped up a higher tier (maybe highest). So, it's highly unlikely there is much difference in ethanol content in regular and premium from pump to pump (pure gas excluded).
Bottom line: there really isn't any unanswered variables. Heating content of the gas is the driver, not octane, with respect to FE.
"Will premium gasoline give better fuel economy than regular?