Feeling your pain... are you getting black soot around the tailpipes like it is burning rich? I guess if I was doing all stop and go driving I could probably get lower mileage... maybe the only thing saving me is the highway.

I put a K&N drop in air filter in the day after I bought it... never thought it would increase the mileage that much but may be worth a look if you haven't done that yet. The part number is hard to find but I found the 33-2063-1 was used in the STS and the numbers crossed back to the GM and AC Delco filters for the 2004 GXP.

Good luck to you!

I know this is gonna get moved, but if you are looking for a 33-2063 K&N drop in filter for your GXP I have a new one in the box. Bought it thinking it was the one for my parents 1993 Park Avenue Ultra and found out other-wise so now the Park Ultra has a CAI, and I sitting on a K&N filter for a GXP.