Mix it 102 and 91 =96.5, ,, 100 and 91 = 95.5 I use 93 and 100 = 97.5 most of the time i run 93, when at the track i add the 100.
the 100 is safe for our car (unleaded) if you run leaded you will burn out you cats and may burn a valve, use the 100 unleaded mixed or not,

in fl. we have gas stations that have 100 and E-85 at the pump,, look into E-85 if you can get it ,,lots of changes to do to the car if you go to e-85 jim