Here you can find information on how to get to Ancient Olympia by car via Allagi.
Athens to Olympia via Tripoli and Megalopolis and Allagi (296km). This route will take you pass the Artemisio tunnel, and Tripoli, continue towards Kalamata. This trip is demanding from to Allagi until Kalo Nero where you turn right towards Krestena. Follow the signs to Krestena and then to Olympia (Please note that some GPS software implementations have different names for Ancient Olympia. It might be spelt Ancient Olympia, Archaia Olymbia, Olibia depending on your GPS maps provider).