If you get better gas mileage at the expense of performance, does that suggest that 87 would be good for a long road trip where power isn't a concern? My fuel door says "minimum 89 octane"... so I can't imagine that I'd be damaging my engine if I used that.
On the other hand, if I'm using the "fuel economy" mode on my JB4 tune, the timing is probably already adjusted without needing to change the fuel grade...
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2008 135i Montego Blue sport pkg, premium pkg, Logic 7 sound, iDrive/Nav
Mods completed: JB4+ & FBO || LSD || BMW-P suspension + F/R sway bars || 18" Breyton GTS-R || M3 front bushings || Vogtland camber plates || BMW-P rotors + Stoptech pads |||| Acquired, not installed: BMS WW Meth kit || Logic HVI intake || Mishimoto OCC |||| Shopping for: Summer tires
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