OK - so a few weeks ago I got the car out for the first time this year. The speedo worked fine for about 100 yards, bounced, then went dead. Never came back. Today I replaced the speedo (a planned change out), and the speedo cable. The driven gear was replaced last fall when I was calibrating my other speedo. I quickly ran the new setup to make sure it worked, and it was smooth up to the 80 mph mark my drill would take it to. Everything fit together today as it should when installed. Got the car off the jacks and went for a ride and it was deja vu. The speedo worked for about 100 yards, bounced around and stopped working. So I'm not sure what's wrong or what to check first. I didn't troubleshoot the old set up because I had already planned to replace it. Where would you start?

 

Look at the drive gear on the output shaft. Likely it's excessively worn.

 

Which would be my luck since I have a 4-speed...

 

It was the driven gear - chewed to a nub. I know the drive gear isn't the right one - I think in trying to get the mph correct I matched up two angles that weren't compatible...