yeah i also get around 280-300 on 14 gallon of gas. I4 Auto 6gen and I have a lead foot. I drive 1/2 on highway 1/2 city i never exceed 3.5rpm usually on the freeway i am @ 3k rpm cruzing around 90mph only use 87 octan and change my oil every 3K plus use injector cleaner every 6k. only mod is K&N air filter.

 

wrong octane gas will make that much of a difference as well... now you tell me which is more common? O2 sensors don't go bad that often.

 

try some bg44k fuel system cleaner. $20 but that could solve your problem?