Are you really driving all the way to Savannakhet ?
Parking up at Mukdahan and getting the bus over is far better.
There is also a very good bus service, the big yellow bus, that travels from Rayong to Mukdahan and passes through Buriram. There are two services. One actually comes into Buriram town centre and then goes to Satuk and up through Roiet to Mukdahan. The other one stops at their new bus station just before Nang Rong and then goes to Ubon and through Amnat Charoen to Mukdahan. Both take about 6 hours.