I just crossed yesterday from Las Americas to Sabana Grande de Boya. Total about 55 kms. Out of that, about first 15 are fully paved. After that, about 35 kms unpaved, but hard and leveled soil, some cemented, some not. Quit an easy passage, no detours, about 65-90 km/h can be maintained depending on the area. From Km 50 (approx.) onward it's fully paved up north.
I went yesterday at 9pm !! and it took me about 35-40 mins from Las Americas to SGB.
Beware - large portions of the unpaved areas have two-way traffic in one lane.
The ride was smooth for the most part and I have no SUV.