There are no "Car Hire / Rent " places in Diafani but there are several "Rent a Car" agencies in the capital, Karpathos.

The roads in the south of the island are mostly well surfaced but the mountain road from Spoa to Olymbos is still mainly unsurfaced and rather bumpy.
The car agencies in the capital may ask you to pay an extra insurance if they know you are planning to use their car to drive to Olymbos and Diafani.

You can however get to Diafani by local transport (boat or taxi).

When you are here you will soon see that the many lovely paths are best for the simple enjoyment of walking through pine forests and nearby olive groves and for finding small bays and pebble beaches for a quiet swim.