100,000 miles at a straight thru average of say 16mpgUS, this is 6250 gallons of gas.Up here 91averages about 20cents a liter more than 87, which I think maths to about 73cents a gallon US difference. What is your actual at the pump difference?
But at say 73cents, that maths to $4550USd, and about a free new engine at 100,000....... or a freshening a lot sooner, probably 4 of them,lol..
This was the principal reason I chose aluminum heads; to run 87E10 at WOT. The fuel savings alone paid for those OOTB Eddies a very very long time ago. I think I broke 100,000 in about the eighth or ninth year, which coulddabin 2009.
If you get less than 16mpg,of course, the savings will be more.
If you can get more than 16 mpgUS, with a hot 360; hey man, congrats.