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your engine was designed for 87. it won't advance the timing. The car nornally runs full advance until it senses knock then retards it.
If Ford could easily get an MPG improvment by specifying 91 octane, they would just to help their CAFE fleet mileage.
also, do a quick match calculation to see how much MPG imrpovement you would need to see tust to break even on the more $$ gas.
summary: don't bother
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