This is a very simple calculation.
Start by determining tire rolling diameter. Example
235 50 18 tire. 23.5CM / 2.54 = 9.25 X .5 = 4.125 X 2 = 9.25 + 18 = 27.25 Diameter
27.25 X Pi = 85.6" / 12 = 7.13
5280 / 7.13 = 740 tire revolutions per mile
740 X 3.55 = 2627 engine revolutions per mile
766 X 3.31 = 2449 engine revolutions per mile
If you travel one mile in one minute that is 60 miles in one hour. Thus this is the number of engine revolutions per minute at 60 MPH.
These calcs are made with top gear being 1 to 1. You will have to factor in the overdrive ratio of fifth gear to get it correct. I THINK that the top gear is .67, thus
.67 X 2627 = 1760
.67 X 2449 = 1640
To get 70 MPH RPM's, simply multiply by the ratio of 60 to 70
70 / 60 = 1.166 Thus 1760 X 1.166 = 2052
NOW, all that described, why are you asking this question?