Hi guys i am new to the forum also i just picked up a 68 convertible and i need help with my speedo. It jumps around 10 to 15 mph up and down all the time regardless of my speed. any ideas on where to start???
also the rear is not original its now a 373 what color speed gear do i need on the 4spd to correct the speedo when its fixed. Thanks

 

needle is jumping from a bad cable imo. Check to see if the cable is melted/damaged. It sits close to the exhaust

 

The cable could be gummed up inside the housing. Disconnect the end from either the speedo or trans, pull the cable out, clean it real well with brake cleaner, lube it up with a graphite based speedo lube, slide the cable back in, and hook it up. Check to see if the cable is frayed at all while it's out too.

 

You will need to change the drive gear in the tranny to correct your mph problem if you switched rear end gears. Wont help the jumping problem though.

http://chevelleengineer.home.comcast...eer/speedo.htm