I find 89 octane gives me a better mileage/price/power balance for the average run of the mill cars I drive. The 87 is cheapest but I can feel the performance loss though mileage is better than 91 octane. I get better power with high octane but my mileage drops and the price is expensive.The mid grades are “middling” expensive but I see better mileage as the better power seems to provide more efficiency per mile which negates the extra expense per gallon as compared with octane 87. Still not as much power with 88-89 compared with higher octanes but definitely more oomph than 87, especially on hill climbs.
I have a 2018 Mercedes GLC 300 - has a turbo and they say not to put under 91 Octane in it..I decided to try 89 octane because some folks at the dealership said one could “get away with mid-grade”....been two weeks and 450 miles or so and so far no perceptible difference except I seem to be getting about a half mile to the gallon better - of course that’s a possible ‘glitch” in the figuring with different driving conditions from time to time....no pongs or other signs anything is unhappy and if there’s a power difference it’s too subtle for me to pick up on...and about 30 cents a gallon cheaper than hi-test....we seem to have similar findings.