i'm driving a 2016 ford edge with the 3.5 V6. i've filled the tank with all three grades of gasoline and definitely see a 10% improvement in mileage with the 89 over 87. not much difference when i go to 91.
the 10% improvement is empirical whereas the power is only judged by feel which could be my wishful thinking. 89 is a tiny bit less than 10% more expensive so it doesn't make much difference except that i can go a bit farther on a tank.
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MY 2000 S, Ocean Blue, Metropol Blue, Savanah Beige.
Bought June 2000 - Sold May 2010