Recently I've been getting 12.9 MPG in my 2.5rs, it's completely stock exept for the K&N Panel Filter, I get a CEL for my knock sensor, it comes and goes, could this be the problem? or maybe my o2 sensors are going?
Any CEL will dramatically reduce your MPG's. Clear it and get it fixed and you should be fine. Other than that, check the normal stuff... tire pressure, plugs, wires, air filter, fuel filter, pcv etc etc.