Tachometer Reading Low

[schlett7nu1]

At the end of last season my 8000 rpm tachometer on my 70 Challenger R/T 440 began reading low.  It reads about 600-800 rpm low.  I noticed while driving down the road it suddenly dropped 600 rpm from it's normal readout.  This behavior has continued for the two times I've driven the car this year.  All other gauges work properly.  Anyone seen this issue before? 

[dodj]

The 50 year old tachs are notoriously inaccurate. IMO, replace the guts with an rte kit. there are POT's in the tach to adjust the tach reading but you need to remove the tach to get at them. So..if you are in that far already, might as well put new, more accurate guts in.
https://www.rt-eng.com/index.php/67-74_Tach_Board_with_Calibrator_Built_In.html

[JH27N0B]

Since I restored my T/A with stock ignition, it's factory tach reads 1000 rpm high!
And my 71 V code with 4:10 with electronic ignition has its factory tach jump up and down from 3000-4000 when driving 60 on the highway so I don't know what rpms to cite when I complain about how high it revs.
I wish I knew about tachs so I could troubleshoot mine to be accurate!