Figure 12,000 miles a year, divided by an average of 20 miles to the gallon is 600 gallons of gas a year. Average of twenty cents difference between the 87 and 91, where I live, and you'll spend approx. $120 a year more on the 91. Ten dollars a month to use the fuel the manufacturer suggests equals no-brainer. (Assuming I didn't screw up the math.)
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