Send help! Speedometer is broken!

[Biggie]

Hey all,
This morning my ‘87 2wd 2.9L Ranger’s speedometer began displaying a massively high reading. I was going 35ish, but according to my truck it was double that. So… what now? I couldn’t find any guides for Gen 1s with this particular issue, so do any of you know where I should start troubleshooting? Any help would be greatly appreciated.

 

[RonD]

It reads like speedometer gauge issue, but..................

On the drivers side of transmission's tail shaft is the speedometer cable hook up, one bolt holds it in place
Remove the bolt and pull out the speedo cable with its Driven Gear

Will look like this: https://www.therangerstation.com/tech_library/images/speed/speedocable_withgear.JPG

That gear meshes with a worm Drive gear on the output shaft of transmission

You can hook up a reversable variable speed cordless drill to the speedometer gear/cable and spin it, speedo needle will only go up if spun in correct direction, same as no speed shown in Reverse when driving, doesn't hurt anything so if needle doesn't go up reverse drills direction

Should get a nice slow response as you vary drill speed, no wild swings, or jumps in speed

Obviously if the gear looks damaged that would be an issue, but usually cause no speed shown or jumping speed needle
Bad speedometer cable is the same, jumping needle or no speed shown


These Mechanical Speedometer gauges work by the speedometer cable spinning a magnet inside a metal "cup" the spinning magnet causes the cup to turn a bit in the direction of the spinning magnet, the needle is attached to the cup
There needs to be a "hair spring" holding the cup from spinning too far this is basically the "calibration" for magnet spinning RPMs and cup/needle movement

Seen here: https://cdn4.explainthatstuff.com/how-speedometer-works.gif

If the hair spring breaks then nothing is holding the needle back, calibration is lost, and shows a higher speed than actual

 

[SenorNoob]

That's some nice info.

Makes me wonder why mine has similar symptoms when cold (roughly around 25F). Always kinda assumed it was some lube freezing up. Clears up after about 30 seconds.