I'm not sure how to explain this, you two just need to get married. I think I have menitoned this before, but sheese....
Honestly, and I will be happy to do some documenting of actual fuel consumption and at what degree incline I have to down shift just to get up the hill with 87, and even 89, but when I put 93 in there, she zooms around like I've got a Texas tail wind and runnind down hill wehn I am going up a 6% grade or whatever.
Additionally, I don't sit down with a pen and paper or one of those high-tech california calculators to determine how many miles I get for how many gallons.
I use the tank rule. I get 250 miles per tank with the 87, about 275 with 89 and ALWAYS over 400 with the 93. That is enough of a difference to me. Not all 4.0L's are the same.
BigJim, I'm not one of those big spenders. Look at what I just posted. 400 miles per tank vs. 250? I would be a "big spender" if I were to put the cheap stuff in there, and would wear out my clutch with all the hills around here!

SO, Back to the origional post, it varies dude, by gas grade, driving conditions, driving habits and the condition of the engine and ALL of it's components. Good luck and I hope that you have gotten something out of all this....