I believe you need to try several tanks on each fuel and average them together. Without making a timing adjustment or such to optimize for the use of a particular fuel, there should be no difference in energy availability. A higher octane fuel burns slower, preventing spark knock. There is no difference in the energy content of the fuel.
I wonder if the 89 octane is indeed preigniting and you just can't hear it. The preignition would increase fuel consumption as well as eventually destroy the engine. There are many things done in the design of the large body cars to decrease engine noise in the passenger compartment. I wonder if it would be possible to not hear spark knock.
Just my thoughts, have a great day,
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in