My 2007 Yaris S doing 350,000 miles in less than 3yrs.

[bronsin]

The source of your pinging is carbon deposits on the cylinder head/pistons. You might consider having the head removed and decarbonized. Also ask them to check your valves and guides for replacement. (valve job) You really want to consider replacing the valves at this point because its POSSIBLE the head of one of them (probably an exhaust valve) could break off and smash up the engine. Your mileage decrease is probably due to valve sealing issues. I also have to believe your timing chain should be checked/replaced too. If it breaks it will destroy the engine. BTW have you had the ball joints/tie rods/steering rack checked? Any failure here and you crash/die. Good luck with it!