Hi folks!
I got a 1991 Ranger 4x4.
Recently I noticed that the speedometer shows almost twice that of the actual speed. However, the odometer was working fine.
What would cause this error? Most of the components of the speedo are electrical, and I'd expect them to deteriorate and increase in resistance, and making the speedo show slower ... can't think of a reason that it's showing faster.
Thanks for your insights!

 

In a 1991 all the parts are mechanical, only cruise and computer use VSS(electrical speed sensor)

Speedometer cable was used for speedo and odo up until 1994, in 1995 all electric was used.
The odometer is a direct mechanical drive from speedocable.

Speedometer isn't direct drive.

So it is the speedometer that is the problem and it isn't a common problem but it does happen.
Most likely cause of your higher speed showing is the "hairspring" is broken.

The Speedometer cable spins a magnetic wheel, the speedometer needle is attached to a Cup that is over the magnetic wheel but doesn't touch it.
As the magnetic wheel spins it trys to spin the Cup along with it, the hairspring holds the Cup back from spinning, this is the calibration for the speedometers needle, so the magnetic wheel pulls on/rotates the Cup raising the needle on the gauge as allowed by the hairspring.
If hairspring is bad then Cup can rotate too much, calibration is lost.

Best option is to find a pre-1995 Ranger speedometer to replace yours

 

Thanks Ron!