I don't know what transmission you are running, but for argument's sake we'll say it has a .759 5th gear, which I think the VG trannies do?? We'll also say you have 205-50-15s, which have an approximate diameter of 23in.
With the 3.9 you are spinning 3019 rpm at 70mph in 5th
With the 3.7 you are spinning 2864 rpm at 70mph in 5th.
Total drop of 155 rpm. You also gain about 7mph in top speed at 6000 rpm (146mph vs 139mph)