There are just a few car-rental firms on El Hierro and the number of cars is limited, so book ahead. Car-hire companies will usually deliver your car on arrival at the airport or port. Cicar is a reliable Canaries-wide choice.
There are only three petrol stations on the island: in Valverde, in La Frontera and on the HI4 to La Restinga.
There are taxi companies in Valverde and La Frontera; it's usually best to book ahead as drivers and vehicle numbers are limited.