RPM not right
 

Hey Guys

I've got an 86 K10 with a 350 I have a Tach on it and at 75mph I used to run 2100RPM now its showing anywhere from 1600 - 2100 RPM I've had 3 tach's on it and they all show the same.

The longer I drive and the more it downshifts for hills and back to OD the closer to 2100rpm it gets any thoughts on how this is happening?