Originally Posted by
747FC
The Saddle Road, as it is called, is the fastest route by far. However, make sure you check with the local office of your rental car company as to whether they will permit you to drive it. It used to be restricted by rental car companies, but recently they have started to allow cars to use it. Just make sure.
Now as to the exact time the journey takes, you should plan for more time because you will want to go slow enough to enjoy the sights, and carefully meander through the winding road. Keep your eyes on the road.
Hopefully, you are not planning on driving this road after twilight.
There are no rental companies with restrictions on Saddle Rd anymore - that went away years ago even before the realignment project. Rental car companies do have standard restrictions on the use of any unpaved roads, which would negate driving up to the Mauna Kea summit, but essentially the summit parking lot is filled with nothing but rental cars.