My 1958 (I know, older than shat you are looking at) has 9R22.5 tires on the back. All stock drivetrain (SM420, 2 speed rear) will turn 4200RPM at 70 MPH. 3600RPM at 60 MPH. Hope that gives you some idea. I dis put a Ranger overdrive in it, that dropped the RPM on the freeway but also robbed some power (even when not in overdrive). I am not sure how much power loss is from adding the ranger unit or from the condition of it. Mine is an old cast iron one and needs to be gone through.
