Argh... On my way out to the store this morning the speedo decides to act funny. For the past 10-15,000 miles the needle would bounce a little bit sometimes over certain speed ranges. (40-45 and 70-75 usually) But now, unless I'm accelerating, the needle wavers and wanders 10-30mph higher than my actual speed! Fortunately I can gauge my speed based on the tach so I know that I am, in fact, going 40 despite the gauge telling me 50-70mph. But I have to be in 5th to do that.

Does this sound like anything in particular? I've heard that a kinked speedo cable will cause the wobbly needle, but I know nothing about how speedometers work so I have no clue what would cause it to swing around like that. (Guess I'll find out! )

Also I noticed that when it's being wildly inaccurate, the needle wobbles around kinda in a circle, when you watch the disk part of the needle in the center.

 

Mine wobbles like 2-3MPH at those ranges, and is worse in colder weather, On a day like zero, I can peg the speedo to 140MPH in 2nd gear!! looks real cool, I use the tach to for speed.

 

had a similar prob myself. then it got worse. it'd say i was doing 70 when i was doing 20. occasionally it'd just spool out. took it all apart and found out that there's a plastic retainer ring where the speedo cable hooks into the back of the inst cluster. it locks the cable into the cluster. when its old, it gets kinda brittle. if its broken, it'll let the cable start to back out of the hole. i haven't fixed it yet, but i gave the cable a good shove into the hole, and it's been fine for a couple months now. i'd actually forgotten about it, so i guess i should go take care of it

 

check the transmission part of it, take it apart and rescrew it back in, if its not the cable or that i have about 4 speedos if you want one. 5$ maybe?

 

I'd put my $ on a binding cable. I bet if you took the cable out of the car and turned it by hand it would be hard to do. Take it out and spray some lube down the inside of it and see if it has a smoother operation.

 

I had an 80 rx 7 do that badly! I replaced the cable, abd had the speedo looked at by a local guy. After that it worked 100%... My 85 has always been 100% on the money...

 

bad cable or connections at the tranny or cluster.

Try re-attaching the cable... if that doesn't work re-place it with a new/good one.

Try and avoid the junk yard ones