Faulty Speedometer
 

Hi, My speedometer in my 79 Z is acting strange... it will stay slow by 10-20 miles per hour and is very jumpy. Is this a cable problem or a gear problem? Oh yeah, the car is all original (no change of gears).

TIA

The speedometer is analog and not electrically driven. It's a straight line from the rear of the transmission to the back of speedometer. It sounds like the little plastic gear in the rear of the transmission housing is breaking and slowly tearing up. You can get a new gear for around 10 - 20 bucks and replacing it will take about 20 mins usually. Best to do it before it breaks off and the speedometer just stops reading. Otherwise you'll need to fish it out of the rear tailhousing. It's not that hard to do but it's hard to see what you're fishing around for in there. If that's it, you can pull the gear housing and take it out until you get a new one.

Des is a spammer, which explains his occasional incoherent responses.
A jumpy speedo could also be from a gummy cable, especially if someone ever lubed it up with conventional oil or grease. You could disconnect the cable at one end or the other, pull the cable out of the housing, clean it well with brake cleaner, lube it back up with a graphite based cable lube, then put it back together. It being 10-20 mph off could be a gearing match issue.

Ha, I remember having to lube my cable. I got crap everywhere.

The only reason I suggested the gear is the bouncing speedo. It reminds me of when mine did it and the gear in the rear housing had broken about 70% off. It was still attached to the spindle so it would read true about 30% of the time when (when it was bouncing) due to engaging the teeth of the transmission's output shaft.