This morning when I went to leave my house today I noticed that my trucks speedo is pretty much doubled. It's a 91 f250 351w with a zf5. I'm clueless as to why it's doing this haha any help is greatly appreciated. Thanks in advance

 

I have seen this when it is very cold out. Then it will work when it warms up in the cab. It sometimes also makes a noise when this happens.

 

That would make perfect sense. It dropped below zero last night and I do hear a very slight noise. It normally doesn't get this cold. Hopefully that's it!!! Thanks!!

 

all three of my 79's and the 88 do the same thing when it gets real cold.
it is a thing with the older cable driven speedometers.

 

That's good to know. Does anyone know how it works that it doubles the speed or makes it faster? Just curious on how that happens

 

never really paid any attention to it except to make sure it spins so the cable does not snap. been there and had that happen way to many times.

 

Most likely the speedometer drive magnet is hitting the needle's drive cup as the cup is made out of aluminum(parts inside the speedometer) and shrinks when cold. Or it needs a good cleaning and re-lubing.


I have the same issue with my 89 F250 speedo too. From the cold days(today was one) I see the speed jumping not as much as yours but I set for 55mph, the speed will ride like 65mph. While doing that I did hear some squeaking noise from the speedo head. After some miles I seen it drop back to normal speed indicating(I verified with GPS on my phone).

 

Is the 91 a cable or VSS? Wasn't this a transition year for the speedo?

 

nope, 91 was last year for cable, 92 was first year for electronic off the VSS.