So for the past 4 years of high school i have run my 96 ssei on 89 octane (go ahead and flame me, it'* the highest octane within 40 miles of me) and so one day about 3 weeks ago i'm in a town with 92 octane so i get some, and after i emptied that tank, i got about 22 mpg.....and i was usually getting around 24.....and after that i put in 89 again, and my mpg had gone down to 20........so what do you folks think is the problem?

i was going to try some fuel injector cleaner, along with maybe a k&n filter, and maybe a gutted air box, if i can find another airbox from a junkyard

i would like to get back to 24 or 25 mpg if at all possible

How many miles on your car and have you changed your driving habits recently?

The number 1 cause of bad mileage is a heavy foot.
If you have more than about 80k miles on the car, you might want to look at replacing your O2 sensor. (Go AC Delco only!)

i have 110,000 miles, and i really haven't changed my habit, maybe a heavy foot her and there, not very often

I'd definately look at replacing your O2 sensor then. Also, have you changed your plugs and wires? You're at that mileage point where a good tune up is in order.

i've been wondering about a little bit of a tune up before i head to college

how much are ac delco plugs? and from what i've read it'* a pain in the *** to put the 3 in on the side closest to the windshield

but maybe i'm wrong...

I did it for the first time last October. I won't say it was the most fun experience, but after doing it once and assisting someone else once, I don't think much of it anymore. One thing that really helped me was removing the strut tower brace (that long bar). Just don't move the car while it'* off.

Plugs ran I think 48-58 bucks for the AC Delco set. O2 sensor can be found here: www.rockauto.com. Cheapest place I found it with shipping factored in.

The answer is obvious. You had your foot planted a bit more firmly enjoying the benefit and newfound power of the higher octance fuel. I only get 15 mpg running 94 octane and when at the track running 110 octane I only get 10.

Mortehl, your nuts lol. My plugs were like $2-3 a piece.

i think he meant plugs and wires....

I suggest for your plugs and wires that you head over to NAPA and get the Belden Premium blue wires for a 1994 Bonneville SSEI (why 1994 -because the 1994 wires are 8mm vs 7.5mm for 96 and they are also cheaper) as well as some Autolite 605 plugs (gap at the stock .060" if you are not modded).
Total should be around $45 US.