Factory Tach VS. Aftermarket Tach

[74Chevy74]

I installed a factory tach dash I bought, and I also left the aftermarket "autogauge" I had installed to compare the two, it seems that the stock one reads about 200 RPM's higher, and react's a bit slower than the other one, is this cause it's stock?

I read all the post's about stock tach's and how to test them, it mentioned if they were slow or stuck that they were going bad, but I just think it's slow compared to the autogauge.

Can anyone tell me what they think? Do I have a bad tach or is it just Stock VS. Aftermarket.....