The only time I have gasoline with octane higher than 87 is a couple of times that the station was out of regular. I am still getting about 450 miles on a tank of gas. As long as the engine isn't knocking you don't need higher octane fuel.
Sometimes older cars will develop carbon deposits that result in a higher tendency to knock. In that case a car that didn't need premium originally could need it as it gets older. The knock tendency could make the ECU retard ignition so much that gas mileage is reduced. Hasn't happened to my Protege yet, though.

 

well now that you mention it there is a sligh knocking noise that my engine makes..or i guess it could be described as a tapping noise..it isnt very loud but you can definately hear it when the hood is up. how could i fix that? the car has 120k on it btw.

~jeff~

 

Just fyi. I got 22mpg today! Vs my high of 33 I think there were a lot more city miles/unwarranted accelerations

it's an auto so that doesn't help I suppose