OK - lets approach this from a slightly different angle.

Currently here in CA, gas is running around $4 a gallon. Typically the delta between 87 and 89 is 10-15cents and the delta between 89 and 91 is 10cents per gallon.

Typical fill up for an X5 is ~20 gallons, right? So you are paying on the order of $80 to fill it up. Extra 10 cents a gallon to go from 89 to 91 is $2. Extra 20 cents a gallon to go from 87 to 91 is $4. How often do you fill up? For me, probably twice a month or less. I think I can swing the extra $8. It's a drop in the bucket compared to what we are paying for the gas. They put the best additive packages in the premium grade fuel - it's worth the extra few dollars per tank just for that, plus these motors are ~11:1 compression and were designed to run on 91 octane fuel. So why skimp? Doesn't make sense to me to even consider it.

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2003 4.6is - SOLD
2003 M5 Imola
2008 Mini Cooper S
2011 Mazda CX9