well your premium and regular are much closer than mine.. the local gas station here the 87 is 3.79 and the 91 is 4.18. it saves about $8.50 per tank to use 87 octane
lets say you get 17 mpg and fill your tank up with 20 gallons each time... you get 340 miles to a tank.. if you drive 15k miles a year.. you should fill up 44 times a year.. you would save yourself almost $350 a year using 87 octane
to balance the cost.. your gas mileage would have to increase by 1.8mpg (a little over a 10% increase) by using premium.. very very doubtful you will see that kind of gain
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