If your car needs 91 Octane, then that's what it needs. You will benifit nothing from putting 100 Octane in a car the only needs 91.

Some high compression engines may require such high Octane... especially older engines that did not have knock sensors to retard the spark advance if the car starts to predetonate. But if your car doesn't need it, your wasting your money by buying it. Premuim fuel doesn't give you better mileage, power, or clean you engine better, unless your car needs it.