My experience with my truck is it pulls all kinds of timing on 87, a bit less on 89, and virtually none on 93. When timing is pulled it kills power/fuel economy so if I can afford it, my truck gets 93.
You may not hear it knocking, you may not feel a difference in performance, but the pcm is most likely pulling timing on 87/89 octane gas.
That is what I believe as well ... I tried the 89 for a year, actually on Sept 11 it will be 1 full year of using 89 on my truck ... so what are my thoughts ?
1. It definitely dropped my MPGs
2. There was noticeable power loss especially on WOT
3. Shifting through first 4 gears in town was sluggish
Even though I don't care for MPG it's nice to have those extra 2-3 MPG if all you have to do is use 91 instead of 89 ...
The power loss was quite evident especially on the highway under WOT ... with the 91 the WOT was smooth and you can feel the full power generated by the engine as a whole .... and WOT is good for engines as it can clean up any build up inside, so any chance I get on the open highway I floor it ...
Also, higher octane from what I have always been told helps your engine run cooler ... so why not use 91, just because of the high price ? lol ... I rather pay a little more now at the pump than having to dump a few thousand on an new engine later on ...