Thank you for the reply!RPM = MPH X Trans ratio X final drive ratio X 336 / Tire Diameter
Assuming a 4 speed auto with 1:1 4th gear and 2.65:1 final drive ratio and stock tire diameter of 25.7 inch
at 56 MPH I calculate 1940 RPM
at 80 MPH I calculate 2772 RPM
As Alien said, sounds about right. There will be some error in the speedo MPH and maybe even some error in the tach RPM.
I found the formula here
Calculating mph/rpm/Gear Ratio/Tire Size | Rare Car Network (rcnmag.com)