I found a web page, Speed versus RPM calculator
that allows you to input different variables to figure out what RPMs you will running with different rear-ends, tires, etc. I have a stock 2012 V6
with a 2.70 rear end. At 80 MPH my tack says 2K RPMs. If I put in a 3:55 rear end this web page shows I will be running 2,650 RPMs at 80 MPH. Does this sound about right?