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Somewhere west of Kipahulu visiting center, my rental car was not allowed to drive on that 

stretch to get to Manawanui Gulch. Some part of road in this portion is very, very primitive

comparing to the road one gets to Hana, the view is splendid (i.e., worth the trip).

You can check google map to get some basic idea (e.g., from Alelele falls viewpoint and west wards,

roughly this was where we U-turned)

But the risk, e.g., one blows a tire and/or need a tow-truck, is also a heavy consideration.

Good luck