Assuming you have a .72 overdrive in the tranny and a tach to go off of
Highway Gearing Highway Gear Ratio 3.51
Engine Speed (RPM) at 40 MPH 1524
Engine Speed (RPM) at 50 MPH 1905
Engine Speed (RPM) at 55 MPH 2095
Engine Speed (RPM) at 60 MPH 2286
Engine Speed (RPM) at 70 MPH 2667
Engine Speed (RPM) at 80 MPH 3048
Engine Speed (RPM) at 90 MPH 3429
Engine Speed (RPM) at 100 MPH 3810
Using this calculator
Gearing Calc
You gonna give us a nice video finishing off the 31s before you put on larger tires?