I'm having issue, and have had issues, with noise from my speedometer cable and inaccurate readings on my speedometer. I'm not sure what I need to fix this, in the past I switched to a larger wheel/tire combo from the stock which made me a constant 7mph slower than what my speedo was reading. When it gets cold although it may sound weird, my speedo goes crazy and there's a lot of what sounds like grinding or moving around. I was wondering what I need to fix this whether I need to fix the gear, cable, or replace my speedometer. I have a 78 Nova with a 250 L6, of little consequence but the trans is TH-350 so I was wondering if I need to replace the cable, what length, if I need to replace the gear what spline/color. Thank You.
--------------------"If it breaks it was good, and if it doesn't break it's bad."1978 Chevrolet Nova two-door hatchback
From what you described, I say you need another speedometer. I can't help with the part about correcting the speed, don't know which gear, etc. But if your speedometer is jumpy and or noisy with the cable in good condition, routed properly and lightly lubricated, then chances are its the speedo head. While there is such thing as speedometer lube and speedometer rebuild services, I always just go for a salvage replacement. Sometimes I win, sometimes I lose on that.