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Curious about this very same question, i started a spreadsheet with mileage & fuel usage. Overall (~40% highway / ~60% surface street) i get 26.752 mpg on 87 octane fuel and 26.274 mpg on premium (usually 93 octane, but at least 91 octane)in my n/a base 1995 850 with manual transmission, so "mid-20s mpg" on the glaciated flatlands of Chicago. I found it very interesting that i get better economy from 87 octane than premium. May be due to the slight boost in performance with premium makes me mash the accelerator more. Also, fuel here is 10% ethanol.
YMMV, obviously.
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