Ok,

If the engine is rated for 91 then I do believe it will generate better gas milage, my point was merly don't run on higher octane then what your car is listed for (many people run 91 or engines listed for 87 and that will not make a difference).

Modern engines when they detect lower octane gasoline will "retard" the timing as to avoid pinging. I could see how this retardation could go over board and would actually make it use more gasoline.

it will be interesting to see what happens if/when you try 91. I suspect 89 could be the best compromise.