The best replacement for 91 octane is 87 octane.

The best alternatives to 91 octane are rated as following:
★★★★★  87 octane
★★★★  93
★★★★  89 octane
★★★★  e85
★★★  95 octane
★★★  94 octane
★★★  93 octane
★★★  87
★★  e10
★  95
★  98
★  100 octane
  92 octane
  94
  89

Your alternative for 91 octane is not listed? Suggest!
Speaking about 91 octane, one probably shouldn't forget about 93 tune.

Other search terms related to 91 octane and their ratings are:
★★★★★ 93 tune
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91 octane
87 octane
93