Road conditions on Koh Tao range from basic concrete to interesting!
Most main areas of Koh Tao have concreted roads, but you still need to watch out for potholes on bad concrete and on dry days (the majority of the year) concrete can be covered with dust. Your wheels can easily spin and you can lose control and fall off even when driving slow.
The "interesting" roads can range from dirt tracks to roads that resemble a dried up river bed.
Because of the lack of drainage, dirt road conditions can change significantly after heavy downpours.
If you are renting any sort of vehicle we would suggest that you check conditions prior to going to the more remote areas around the island. Many of the motorcycle renting companies, now offer insurance for minor damage caused to your motorcycle, we recomend you take this if they offer you it, as a small accident on a rented motorcycle usually costs in access of 30,000baht in charges. It is law in Thailand that every one should wear a helmet and police do occaisonaly enforce this. Ok a helmet may mess up your hair but it could save you from a seriouse injury as well, so please wear a helmet.