Yes, '91 should have a speedo cable AND a VSS(vehicle speed sensor).
Look at the drivers side of trans toward the rear, you will see the speedo cable and then the wires for the VSS with it.
If you look here:
http://www.therangerstation.com/tech_library/speedometer.shtml
3rd and 4th picture down you will see the speedo cable hook up with VSS built in.
I think these were added in 1990, VSS is used for computer and cruise control.
If your doesn't have this you can add it, the mounting is the same with or without VSS added.
And if you have changed tire sizes or axle ratios you can change the speedo gear(in picture 2 or 4) to get correct MPH back
You will need to check with the gauge maker on what types of speed sensor signals it is compatible with but 8000 pulse per mile and Ford signal is fairly standard.
Cap the speedo cable hole in VSS to weather proof it.
Wiring colors vary but VSS used Pink/Orange stripe as Ground, Grey/Black stripe is signal
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