Plugs and wires should be replaced, depending on mileage. Only use motorcraft wires. Plugs, your preference as long as your preference isn't Bosch platinums. I like NGK.
But your mileage problem (assuming your tune up is good) is probably a dirty MAF sensor. Take the MAF sensor out, or at least get access to the front side of it, and spray it clean with Mass Airflow Sensor cleaner. Do NOT get aggressive with the little wires, they break easy. But its those little wires inside the MAF tube that are the issue. Over time, they carbon up, which insulates them, and makes them read wrong.
If that doesn't fix it, you need to look at the front O2 sensor trims with a scan tool. Or just buy new Motorcraft O2 sensors for the front. Even if they aren't "bad" according to the computer, they still wear with age and will rob mileage. Nothing helps mileage like new O2s.
