I tried 89, and the gas mileage difference was very noticeable. Car lacked punch too. I use 93, and when I get to half, I fill up with 89 making a 91 blend. I then run that tank fill with 93, then repeat. Savings, without sacrificing performance or gas mileage.
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'07 335I w/ Mods
'13 X1 Stock
'11 X3 K&N
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