Join Date: Mar 2002
Location: Austin, TX, USA
Re: Bouncy speedo
Steptoe covered the service aspect pretty well. As for the 5mph difference, you'll probably need to change the speedo gear(s) in the transmission.
There is a plastic (usually) ring gear on the tail shaft of the transmission. The speedo cable mates with a plastic pinion gear. One screw / bolt will allow the speedo pinion assembly to pop out the side. Note the color of the ring and pinion gears.
Go to your local chevy dealer and tell them the gear colors you have, the speedo error (5mph) and transmission style and they should be able to give you the correct gears to make it work.
There are a few different pinions that mate with any given ring, but depending on how far you need to go, you may have to replace the ring gear as well. That involves removing the drive shaft, and the transmission tail shaft housing and slipping the new ring on. It's not a big deal, but does require some basic mechanical skill.
Steptoe also mentioned that the gears may be mismatched. As I was saying about the compatability of rings and pinions, someone may have slapped a different pinion in there trying to accomindate a rear end gear ratio change and got them mismatched. I think your GM dealer will be able to tell what's what with the color info you give him.
If you know the rear end ratio and tire heigth, that may be useful, although usually just knowing what you currently have for speedo gears and the amount of error is enough to get it right.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI