I got new tires on my 1979 Pontiac Catalina today and took it for a cruise. The speedometer bounces around until the car's doing around 40 mph, then it runs more or less normal. What would cause this on an old car?
The speedometer cable is binding up. Usually they can be lubricated rather than replaced as long as there aren't any kinks or severe wear. Here's a forum post where lubing the cable is discussed:
https://www.fordbarn.com/forum/showthread.php?t=168799
Is that cable accessible through the back of the instrument panel, along with the light bulbs? If so, I'll see if I can get up in there and inspect the cable.
Yes, you have to get behind the instrument panel. Be aware that it was common for manufacturers to hide razor blades and jagged metal back there so be careful. The cable unscrews from the back of the speedometer.
The cable itself is inside a sheath and you can bet the decades-old lubricant in there is all dried up.
I'll keep that in mind. Tomorrow I'm going to try and get up in there. The instrument lights are out, and the gear indicator doesn't move with the shifter. So I'll inspect everything in one go.