Living in Texas I've always had 93 octane everywhere...I'm now in Los Angeles for the next 11 months and all they have out here is 91 octane. I found a station with 100 octane at the pump...so how much 100 would I mix with 91 in a 25 gallon tank to stay above 93 octane?
I'd rather mix the fuel that retune for lower octane gas.
The truck is strictly a weekend toy so I won't have to hunt fuel down too often...maybe every month and a half or so.

 

I see your post in the forced induction section on meth. Put the Alky Control kit on and don't worry about the 91 octane.

 

1/4 tank (6.25 gallons) of 100 octane mixed with the rest at 91 octane works out to 93.5 octane. I think you should use 100% 100 octane, raise your compression, or boost if you have it, and retune for 100.