James0598, you have to take the tire diameter into consideration. You are correct that if I do 4300RPM in 4th at 70, then by changing to a 3.9 diff I should lower my RPM to 3900 and go the same speed. A good rule of thumb, however, you can't work it the other way.
If I have 18 inch wheels, and you have 16, I am going to go 56.52 inches (discounting tires) and you 50.24 inches per wheel revolution. What about different size wheels offered by the maker on the same car? Tire aspect ratios?
The standard differential ratios, which can be found on the door jam or engine bay sticker(1986-88) follow are below. All are 4-pinion 8" differentials:
w/o sport pkg :G284 4.300 open
w/ sport pkg :G285 4.300 LSD
turbo (87-88) :G305 3.909 LSD
turbo (89-92) :G315 3.727 LSD
There was also a 4.100, labeled a G295, which was not offered in North America as far as I know.
FYI, My 86.5 NA does just about 70 at 3500 MPH in 5th. This gives it a theoretical top end 130MPH at 6500; but the car can't pull 6500 in 5th gear.