I have a 95 F-150, 5.0L AT regular cab.
Without warning the speedometer became erratic at any speed above 50 mph.

Replaced the vehicle speed sensor (VSS) on top of the differential, (no change).

Re-installed the old VSS, still no change.

Installed jumper on EEC connector (OBD 1), (clipped to fender well in the engine compartment drivers side, corner nearest driver), and read the following codes flashing on the "check engine light":

DTC

Hard Codes
55 - no definition
56 - Mass Air Flow sensor circuit above max voltage.
65 - Overdrice cancle switch open.

Memory Codes
12 - RPM unable to reach upper test limit.
21 - ECT out of Self test range.
22 - BP sensor out of self-test range.


Removed and cleaned the MAF, still no change.

Re-installed the new VSS and disconnected the negative battery cable for about 10 minutes.

Drove on highway for 30 mins (PROBLEM SOLVED)!

I checked the codes after I got back from the test drive and all the codes were still there, however the speedometer problem was solved, so more troubleshooting for a later date.

Thank you YouTube

Reading OBD1 codes

Cleaning MAF sensor