So I have an 86 Sport, and my speedo does a thing where it it spikes up and down +/- 10mph from what I'm actually doing. (i.e. I'm doing 65 and the needle jumps between about 55 to 75 mph constantly)

I'm guessing that either I have something up in my gauge cluster, or the speedo cable is off a tooth or something of that sort?

Anyone have this sort of issue before?

 

The cable has no teeth at the trans side.
It just slips into the gear on the trans.
It has a square-ish tip if I recall correctly.

A couple things you can try is to wiggle the cable at the firewall and the trans to see if it changes anything..or take the cable off at the speedo and squirt some lube into it and let it slide down the inside of the cable.
These suggestions are short of replacing the cable or cluster.

 

I'll try that, thanks!

 

Worst case, NAPA carries the speedo cable and it's like $17.