Plugged cats would surely throw a code and would noticeably affect power and faulty 02 sensors that have a significant effect on air fuel ratio would also surely throw a generic trouble code as well (not manufacturer specific in either case). What is hard to diagnose are manifold absolute pressure sensors (MAP) or mass airflow sensors (MAF) since detecting out of range is somewhat difficult and it takes special driving conditions to get that code much of the time. I believe your Magnum had the MAP sensor but in either case, people that install K & N air filters are usually looking for trouble with those sensors since they tend to over-oil the air filters, which in turn degrades those sensors in particular. So you may be running rich under most driving conditions, thus affecting your mileage, and without a trouble code. More recent vehicles have improved MAP or MAF sensor diagnostics. You might want to spend the money to have a competent tech take an in depth look at the real time diagnostics the OBD II system provides. The OBD II system will hook up to a PC and the techs can look at the outputs of a multitude of sensors and spot the faulty ones. I believe there is a pretty competent OBD II tech in Arizona, but I would have to make a phone call or two during the work week to get his contact info Chris if you would like.
Also, with the hotter weather now compared to when you first got your Magnum, heavy use of air conditioning could easily drop your mileage by 2 -3 mpg.
Steve
