Have a qualified technician scan the system. A good technician will figure out why the mileage dropped. It could be any number of things from a leaky injector or fuel line, fuel pressure regulator, distributor cap, rotor, wires, spark plug(s), MAF sensor, a "lazy" oxygen sensor, just to name a few. Emission control warranty covers far more than just the converter and computer. Any part the computer receives data from, or is controlled by the computer, is considered an emission control part. There are some additions too, plug wires and spark plugs, but they don't want you to know about it for obvious reasons
Don't let 'em stroke you.