Factory tach reading low

What's the likely cause for an unrestored factory tach reading a few hundred RPM low, all of a sudden, having been generally fine a few months ago?

Thanks

 

The 50+ year old electronic components have shifted their values due to age. Will need a new electronic board. I do those

 

Thank you Larry and Scott. So no chance that GM would warranty/replace the 51 year old tach?

Trying to remember the effort in removal; it's been 20 something years since I was last in there. I'll contemplate the exploratory surgery and contact you, Scott, when I'm ready on this.

 

Removal is a pain in the a--. in a 70-72, remove glove box liner, remove dash pad, lower steering column, disconnect speedometer, remove screws from instrument cluster and pull it out enough to get to the nuts on the back of the tach. Another way is to remove the radio and the middle A/C duct to access the tach from the side. That's what I did since I had to remove and replace the front radio speaker anyway. It can be an all day job if you're a slow worker like me.

 

Thank you Ray. Those are generally what I recall. I've been in there in both an A/C and non-A/C car; and at least three times since the mid-80s. We're getting old.