Saddle Road, which is State Route 200 on the Big Island, is now completely paved from the Kona side all the way to the Hilo side of the Big Island. The Mauna Kea Observatory Access Road is also paved between Saddle Road and the Mauna Kea Visitor’s Center. However, beyond the visitor’s center and all the way up to the observatory, it is not paved. It can also be very dangerous. It is forbidden to take rental cars on this unpaved portion. The car rental companies do NOT allow any of their cars to be taken off the pavement, even if they are 4-wheel drive (4WD). Going onto any unpaved area in Hawaii is a car rental contract violation.
I have been advised by Avis, Budget, Dollar, Thrifty and Enterprise that it is okay to take their rental cars on Saddle Road. But not all allow their cars on the Mauna Kea Observatory Access Road between Saddle Road and the visitor’s center, even though it is paved.