Driving Southern Thailand:
A Budget Rent-a-Car "World Class Drive"

This guide describes in excellent detail a circular drive around Thailand's southern region.  It starts in Phuket and drives up the coast through Khao Lak and Takuapa and on the Khao Sok National Park.  From the part it details the drive onward to Surat Thani and the ferry ride over to Koh Samui.  From that point the drive returns you to Surat Thani and gives the details for the drive on down to Krabi - and from Krabi back to Phuket.  The guide notes points of interest along the way as well as important landmarks to look for. 
The roads in the area are all marked in Thai and English and in good condition.  The landscape is stunning and the drive is well worth your while.