At today’s prices, premium fuel only costs about 5% more than regular. Regular has up to 10% ethanol. In my area, Key Largo, I can buy REC 90 fuel - no ethanol, 90 octane - for 5% more than regular. Theoretically, I should get better mileage from the REC 90, but would it be enough better to justify the higher cost. My car Solara averages about 27 mpg.
Whatever your manual says. If it says that higher octane isn’t mandatory, I would experiment and see the mileage difference for myself. You will get better performance only if the car was designed to on higher octane fuel. It seems to be up to the software these days.
The only way to find out is to try it.
Agree with mark9207 to try it out.
Since your Solara currently averages 27 mpg, any increase over 1.35 mpg means it will save you money.
Unless your car requires premium fuel, you should not get better gas mileage when using it. Octane is only a rating of the fuel’s resistance to combustion/volatility. It does not denote the “quality” of the gas or the amount of energy per unit volume that the fuel contains.
Unlike the typical “regular vs premium” questions, the key question the OP is asking is:
Is it worth paying 5% more for a fuel with no ethanol than a fuel with 10% ethanol.
Given what ethanol does to mpg, it’s worth experimenting.