Stock tachometer vs. Auto Meter tach

I recently installed an Auto Meter Autogage in my car and have noticed there is a BIG difference in RPM once the car gets over 3K… Which one should I go off of? I’m certain the tach is wired correctly with the 1PM wire to tge blue distributor wire. I’m a little hesitant to run tge RPMS up that high for obvious reasons… So I figured I’d ask, any advise?

How far off is it? And is it just a lag or is it an actual difference in reading? I’d love to see some pics.

I’ve read on a couple of sights that there is a difference at high RPM. Firstly, my car is am automatic… That having been said.

I set my shift light at 6500 originally because that’s where red line starts on my cluster and where the car normally shifts. It never came on! Tge third time I watched it right before it shifted and it was around 6100 so that’s where I set the light and it barely comes on…

Also, do you happen to know where I should be shifting and where I can safely run it up to?