I've always used 91 octane on any of my M cars (E46 m3, e92 m3, f87 m2c, g80 m3c), since it is what bmw engineers recommand.
I didn't really check the fuel economy to be honest since those are or were performance cars (for their generation).
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Instagram : @gc.g80brg
2023 M3C Xdrive British Racing Green
2009 M3 Alpine white DCT
2001 M3 Laguna seca 6 speed
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