Possible causes and fixes for speedometer jumping to 70 kph as soon as start moving ??

Had been working fine. Sat for couple weeks in temps bouncing from 5c day to -9c night. Morning I went out to go somewhere the front tires of it's fwd were in small ruts of ice from frozen water maybe 1-2" deep. They spun in ice rut so bit more gas got it moving back out. Nothing severe rpm wise. Done similar in past when bit of wet snow and ice there with no problems from that in past. It slowly moves up to 70kph and stays there as soon as I start moving forward even when I'm only at 30 kph. It doesn't vibrate, just stays there whether I speed up to 70 or slow down or hit bumps. But punching dash or on or under steering colum makes it drop about 10 with each punch. Have to do this when stopped to get it back to 0. But as soon as start moving again it does same. Punching it while moving gets the small drops but it slowly goes back up to 70 even though doing only 50-60 kph. It's like it's on a higher gear of it's own. Reaction to dash punches suggests just an issue inside speedometer ??

 

FIXED WITH JUST A TAP AT NEEDLE BASE !!
I managed to get clear plastic of dash out that covered speedo and other gauges. It can be tricky if no obvious screws but instead hidden clips usually plastic. So 1st with slot screwdriver I carefully pried being careful not to crack the plastic, gor the 2" piece all around it out. Then the clear plastic outer edges were visible but couldn't figure out how to get it to come out. It was tight against instrument gauges panel. Finally I decided it's old I won't be selling it so if it breaks bo biggy. Harder prying with slot screwdriver cracked it and I got all odf it out. 1st tried tiny bit of silicone spray thru attached straw to just above center base of needle to see if it work it's way along shft to inner mechanism and get a different result and maybe fix while driving and adding light punches vibrations to it thru steering wheel or dash.
tried few times while driving. no effect. needle still wanted to stay at 75 kmph regardless of speed or stopped.
so while driving I noticed it could easily be moved up or down with near no resistance. but quickly returned to same 75 kmph regardless of speed. slowing and stopping it styas there. can be moved back down but gows straight back to 75 even when only doing 10.
Next on possible easy cheap fixes list of things to try is a light tap at round base of needle in middle. pressed against thumb flicked the index finger nail to that center round part. BINGO !! LOL It dropped to around 55kmph which I figured I was doing matching traffic ahead, beside and behind in 50 zone which usually does about 55ish. Further driving, slowing and stopping, 60 zone, it worked properly. After hour of going here and there getting stuff I hit a few smallish potholes I didn't think would be much of a bump but were moderate. Needle resumed sticking to 75 while doing 55. Simply light flicked needle base with fingernail again and it's been fine again sone. So there you have it !! ;-) If yours is like or behaving like mine which I found out is just cable from tranny, if it's still responding to cable by moving if you punch dash, issue is in the mechanism behind the needle. Lubrication or tapping may be all you need to correct it and avoid many hrs shop bill to take aprt, order new mechanism, install, put back together. It's around 4 hrs total minmium. With the cover off you can easily give it a tap if need be. ;-) I had to come back here and share this as I believe strongly in sharing what may be very helpful or even just amusing or enjoyable for others. Thx to net and subject forums it sure is easier these days to maybe find answers to many questions. ;-)

 

Well done!

 

Was this for your 1994 Pontiac Sunbird mentioned in your profile?