Here is some info from the old Pontiac days.

These are close:

Miles per hour, per 1000 RPM in final drive

3:08=25.6 MPH
3:36=23.1
3:55=22.2
3:70=21.3
4:11=19.2
4:56=17.0

example: 3:55=22.2 x1000 rpm=(3000rpm)=66.6MPH

This was from the early 70s.

Good luck
Tim