So I tried searching and couldn't find anything, so I thought I'd post up for any thoughts on the subject. I have a 1991 Camaro RS with a TBI 305 and 700R4 (original motor and trans). The issue I have is it sat for a couple years registered non-operational, and I just got done replacing the fuel pump (not fun!) to try to get it registered and running again.

After hooking up the new battery (old one was toast), the speedometer began acting rather erratic. I haven't left the driveway, much less done any more than push it up and down the driveway once, and the speedo randomly jumped up and sat square on 60MPH. It stayed there for the most part, until it randomly decided to jump past the top end, indicated 110MPH, and the end of the needle now sits tapped to the bottom of the housing, as if I were going about 140-150MPH. Everything worked perfectly fine before parking it, in fact, once upon a time she was my DD. Any ideas on what to check, or what could be causing the speedometer to malfunction?

 

Might check the electrical connector at the driver side tailshaft and make sure it's connected well. Might also try slightly loosening the screws that hold the cluster in, and slightly wiggling the guage cluster - might have a contact issue on the back of it.

 

Okay, so I got a day pass from the DMV to move the car from where I was living to my parents' house, and on the way, at first I was just cruising lightly, and the speedo stayed pegged out. When I got to the road that is a straight shot to my parents' house, I got on the gas a bit from each stop sign, and after the first time I got on it, ran it up to about 40-ish MPH, and while doing this, the speedo snapped back to zero real quick, paused half a second, and snapped up to my actual speed...it's been working perfectly since. I don't get it, but hey, I'll take it haha.

 

While it's working (ie NOW!), jot down the rpms at 35mph, 45 mph, 55mph, 65mph ... then if it goes out again, you'll know based on the tach what your speed is.