I have a geo metro 94' , 3 banger, and it recently started to ping really badly. Now I have been putting in the cheap gas because of the prices. So I tried putting in some octane boost & it helped alot. But it still does it more than I would like it to. Any other suggestions besides checking timing belt to see if its off by a tooth or so ?

Retard the timing or take the head out and clean the carbon from the top of the pistons and combustion chamber.

If there is a chance the cylinders are carboned up you could use some seafoam. You pour a little down the TBI while it's running shut it off for 5min then start it back up. While you are doing it it smokes like crazy but it cleans out the gunk. Check out this page:

http://www.misterfixit.com/deton.htm

hth

Oh yeah since its a 3cyl. I would only use half the suggested amount.