Make sure the tires are inflated properly, next time you drop the trans pan and change the fluid/filter install a transgo shift kit (less slip for the trans.), clean the MAF (with proper MAF cleaner- NOT brake/carb cleaner), plugs/wires.
Since I drive a LOT of highway in my 00 GT (500mi./week minimal) I went with Irridiums and it runs just fine. See about ~410 per tank empty to full (this includes having a bunch of telco stuff riding around in the trunk all the time). I only do 5 over the speed limit usually and am easy on the gas. Think I can get a bit more out of it once I figure out my "Bank 1 lean" code. I have yet to track that one down...
