Driving from Phuket to Railay
 

We will be staying in Phuket in April, with a hire car for the whole time.Is it possible/reasonable to drive to Railay? How far is it? Roughly how long should we expect it to take?Should we stay the night-if so any suggestions(cheap)? We drove a hire car in Koh Samui last year, so we have a limited experience of Thai driving.

You can only drive to Aunang or Krabi town. From either, you have to take a boat to Railay. There are no roads or cars on Railay. Someone else will have to give you the driving time. It would take at least 3 hours if not more to drive to either from Phuket.

The road is easy, over the bridge on dual highway, veer to right direction Phang Nga / Surat Thani, kms from Patong ( so adjust from where you are driving from on Phuket )just on 95 km to right turn ( direction Surat ) from where road becomes a basic 2 way and from there Krabi signed.
As twofortheroad has said that will get you to Ao Nang, then you have the issue where to leave the car, if for the day just park where you can, but if overnight then the ideal thing is that you should rent from "Budget' and so you can drop car with them ( having booked one out for when you plan to return ), saves wasted rental charges and saves hassle on what to do with the car whilst you go to Railay. If just for a day trip Budget would almost certainly let you 'park it' with them, they are at Ao Nang Villa Hotel in Ao Nang.
Altogether bit of a rush for a day trip, I would overnight, in which case 'returning your rental to budget and then booking another one 'out' will save you rental charges ( depending on the original check out of the vehicle ).
Budget Thailand are excellent and reasonable and ideal in this scenario.

That 95 km is a 'guide' to distance from Phuket to where you take the right hand turn just past Phang Nga, 3 hours all the way is reasonable.
If you need fuel there are plenty, in the south PTT ( Blue / red / white signage ) are best for washrooms / shops / on the way stops.
The stretch after the Phang Nga where is becomes the normal 2 way is fairly flat with a few minor rises, but quite a lot of bends and you will see some hair-raising overtaking on that stretch when people get stuck behind trucks, just take the drive relaxed and easy.
Budget also provide excellent maps.

Thanks for the help. As we have already booked our car, in Phuket, we may change our plans, as I agree,it would be a waste of a days rental,plus the cost of a nights stay.Not ruled it out though-Railay looks very impressive!
Clare