To return to the OP’s opening query, we can begin by calculating engine speed at 70 MPH for the car in question (’70 114C), which, if a US spec car will have a final drive ratio of 3.92:1, and tires of 76.5” circumference.

70 MPH = 6160 ft/min

6160/(76.5/12) = 966 wheel RPM, 966x3.92 = 3788 engine RPM

If the car were non-US, with a 3.69 final drive, engine speed would be 3565 RPM

At 4000 RPM, vehicle speed would be 74 MPH, and 79 MPH, for the two final drive ratios respectively.