My '87 E150 302 van did something really odd recently.
I was at 60 mph and saw the speedometer needle start creeping up. It was at 85 for awhile and still went up and down with my speed but I wasn't going anywhere near that fast. GPS is like 60-65mph.

Eventually I saw it go straight down, like 100 mph. When I slowed down it went back to sane. It seems to be ok now.

I figure if a speedometer gear or cable was messed up it's going to read low, not high, due to some sort of slipping. How could it read high? Is there anything other than the gauge?

I was worried the speed density-based computer would be confused if it got the wrong speed, but I didn't feel anything different and the mileage was typical. I figured the computer has a tach rather than post-transmission speed signal anyways.

 

It could very well be the cable. If it binds it can slip and cause the speedo to read fast.

 

Our '92 E-350 just did the same thing. We went to about 70 in the carpool lane, and it started bounceing around and reading high. Ours read as high as 95.

 

I guess it's contagious.

I don't see how it could be the cable. It's turned from the end by the tranny so if it were slipping then it should be turning slower at the gauge, not faster. I'd guessing the gauge is bad? Geez I don't wanna take the dash apart for this.

 

The cable is basically a giant spring, it is turned by a gear in the transmission. As the spring binds in the cable's sheathing it can cause and erratic display.

 

But it wasn't jumping around as binding and springing would suggest, it just drifted upwards...

 

Is it noisy, or just acting odd? It sounds like a cable. The cable isn't solid, it's a spring. It may just need lubed. Pull the dash out, and then center portion of the cable. Get a tub of white grease and grease the entire thing. Then reinstall, make sure it seats in the gear in the trans. That is the tricky part.

 

i am not sure of all speedometers but the old type used a magnet to transfer energy to the needle so an alignment problem could cause the needle to do just as u say as in the magnet sticking or just closer to the disc!