fdecarlo

Bob, the trek to Mauna Kea involves two segments: Saddle Road (Hwy 200), and the cutoff road up to Mauna Kea's visitor center, and eventually the summit. Now the cutoff road is paved and smooth and nice, all the way to the visitor's center, but Saddle Road is a different story -- especially the western approach from Kona and Waimea. This portion goes through an army base, and the asphalt has been badly chewed up by army tanks and other military vehicles. And to make matters worse Saddle Road is never maintained or repaired.

But if you're coming from Hilo, the eastern portion of Saddle Road isn't bad at all.

As for which rental companies allow cars on Saddle Road, the answer is not many. It'll take some searching, or you can rent from Harper's, or just ignore your rental contract and hope nothing goes wrong. But remember, if you get stuck anywhere on Mauna Kea the rental company will know you've been driving on Saddle Road. It's the only way to get there.

Meesthare

The other problem with the Saddle Road is that there are frequently convoys going to and from the army base. They are long and move VERY slowly. It's pretty frustrating driving behind them, and there is the possibility that your rental car might overheat.