Actually, it's not quite that simple. No doubt the manual will say "Use 91" but "89 is OK" like most Nissan manuals. But, there's more to it. Stick with me until the bottom line.
Here's how it works: When you accelerate, there's a chance for pre-ignition (knocking). The knock sensing mechanism is constantly checking for knock. If it detects it, it will retard the timing (in a Nissan) and stop the knocking. So, within reason, _no_ gas will cause damaging knock or engine damage.
Why do we care about octane ? Retarding the timing will reduce available power in the engine. You will have less power overall. If you are going up a hill, or pulling a trailer, or just entering a highway, you may need to press harder on the accelerator. So, gas mileage will suffer. At the upper end, you will simply have less power.
So, does it matter ? You will get slightly poorer mileage _depending_ on the exact driving conditions that week. If you are doing a lot of mountain driving, or pulling heavy loads, or just like a lot of power, you may find it somewhat diminished. Whether or not it is more economical to use Premium or Regular is very arguable.
Here's a sample calculation:
12,000 miles @ 20mpg = 600 gallons of gas used; at $1.59 for reg gas you pay $954 for gas for the year. With Premium gas at $1.65, you pay $990 for the year. That's a whopping $36 a year more for Premium. Whoopee. Even if Premium is $.20 more per gallon (and I doubt it) you still only pay an extra $120/year for Premium. And, we still haven't figured in any extra mileage due to using premium. If you get 22mpg using Premium instead of 20mpg because of the higher octane, then you only use 545 gals of gas and the Premium costs you a total cost of $900 -actually $45 *less* than regular. If you get even 1 mpg more with the Premium it costs the same as regular on a per year basis - and you get all power Nissan designed in.Bottom line: These number are speculative. Fill in your own. The fact is that the difference in gas cost is going to be +/- $50 to $100. Now, check the price tag on that new SUV and tell me again that $100 per year really matters either way.
Bob