I've driven the road to hana probably 40 times, majority of those times in a Jeep, just remember, if you go the complete loop and anything happens, you are on your own, the rental companies make you sign a waiver when you get your car that says if you go around the back side, where Hana Highway turns into Piilani Highway then they wont come get you or cover any damage to the vehicle and can be liable for the total cost of the vehicle. This is also true on the north loop out of Wailuku going around to Kapalua. I never worry about it, but with that said there are a few areas around the backside with some good trails going down to the ocean with some good obstacles, other than that, not really much for wheelin there that isn't on private property.