Driving can be a good way to get around the larger islands of Viti Levu and Vanua Levu.
Like Australia, New Zealand, Ireland, the UK and most of Asia, traffic in Fiji drives on the left. The maximum speed is 80km/h although this is only 50 km/h in built-up areas. If you drive in Fiji, you'll need to deal with pot-holes, crazy drivers and assorted animals.
Rental cars are relatively expensive although it can be worth the cost if you split the cost among several other travellers. You should check whether the rental car company allows you to take the car on ferries to the other islands and drive on unsealed roads as this can restrict where you can travel.