When I purchased my car, 1965 Cutlass F85, the speedometer needle was not accurate. It just kind of dances around above 50. I changed the cable as best as I could given that the drivetrain is not factory; 455 w/ TH400. I am getting the same 'dancing speedometer needle' as before. Is there a way to fix this or does this have something to do with the speedometer itself? I am currently guesstimating my speed with my tachometer. I made a spreadsheet showing approximately what speed I should be driving given the rear gears, tranny gears, and tire size. Thanks for any help.

 

what you are describing sounds like a bad cable and usually changing it fixes the problem. Since the drivetrain is different and you said "as best you could" you are hinting that it may not be the correct cable, right? That might be the problem or it's possible the cable housing is damaged and causing the problem. Maybe the replacement cable doesn't fit the cable housing correctly too, there might be different gauges. Just thinking out loud....