If you throw a frog in a pot of boiling water, he'll jump out. Put him in some nice cool water and slowly bring it up to boil, he'll swim around dumb and happy until he croaks. Or so I've been told.
I was that frog recently.
My last two fill-ups have not reflected the MPGs I thought I should be getting. I was wondering if the new spark plugs I put in were to blame. Then I checked my tire pressure before leaving work. Yikes. They were all lower than where I like to keep them, and the front left was down below 30 psi. I put them back up to where I want them. (I'm NOT posting this story to spark yet another PSI debate...) Sure enough, on the way home the Vibe glided further and required less LOD on the SG to maintain speed, ie; like it used to! I had slowly gotten use to shorter glides and higher load readings over a period of time. Grrr.
Moral of the story; Don't be a frog in a pot. If your car doesn't perform like it used to, investigate. Check your tire pressures!