Gas Mileage Problem in my 01 GT

It runs good?No codes? All factory emissions etc in place? Is the speedo/odometer reading correctly after the swap? What is your driving style?

+1 regarding IF the speedometer is accurate. The PCM uses the car's speed sensor signal for multiple purposes. The speedometer needs to be accurate.

If this were my car I would be very concerned about the stability of the cam and CKP sensor. What can "cause" an unstable CAM and/or CKP sensor signal?

• a bad alternator diode (alternator problem).
• a loose CKP sensor
• A broken crank damper outer ring
• an improperly torqued crank damper bolt.
• underdrive pulleys.
• Engine ignition wiring problem. Check spark plugs and COP insulator boots.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

Having access to a high end ODB2 scanner capable of data logging could make short work of this problem. Need to know things like:

• Fuel pressure
• Long term fuel trim
• MAF flow
• Misfire data

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

>>From Ford service manual.
P0320 - Ignition Engine Speed Input Circuit Malfunction

The ignition engine speed sensor input signal to PCM is continuously monitored. The test fails when the signal indicates that two successive erratic profile ignition pickup (PIP) pulses have occurred

• Loose wires/connectors.
• Arcing secondary ignition components (coil, wires and plugs)
• On board transmitter (2-way radio)

The DTC indicates that two successive erratic PIP pulses occurred.