My 03 Sedan gas mileage dropped from consistently 21.5 mpg to consistently 19.5 mpg about a year and a half ago. I've tried changing several things to see if there was something simple like changing gearbox oil (6MT), differential oil, proper tire pressure, oil change, and a clean air filter and nothing seemed to help. My last thought has been maybe the spark plugs we going bad (were last changed at 105k, now at 190k). I changed them and don't have a good metric of what a good or obviously bad spark plug looks like.

I've attached a photo of the 6 plugs. Any thoughts?

BTW, the number 4 cylinder (right side middle) had oil around the plug, so i'll be replacing that valve cover soon.

 

They don't jump out at me as being "bad", but def wearing out.

You do have 190K miles on the car though. It is entirely possible that the engine is just starting to get "tired"