Due to problems in ex-Jugoslavia, the only route for cars into Greece is via Italy. It is recommended that Ancona is used as a port of exit, not Brindisi, because fuel in Italy is very expensive (1750 Lire/liter) and the road tolls prohibitive. It costs Lire 69000 until Brindisi, which is about BF 1300, for the one way toll!
There are plenty of ferries from Ancona, with the biggest Venizelos M/V taking 3000 people and 1200 cars. For a family with a cabin, the fare is about BF 18000 return.
The way to go would be to leave early in the evening, take E411 to Luxembourg and get petrol there. Luxembourg is one of the least expensive places for petrol in Europe. One liter costs LFR 23 (unleaded). Compare that with BF 36 in Belgium!
Follow the road to Metz and then Saarbruecken passing the river Rhine at Strassburg and then getting on the Autobahn direction Basel. You must have a Swiss highway vignette on your windscreen (Sfr 40) current for the year (includes January of the following year). From Basel, you head on direction Luzern and then passing St Gotthard tunnel, you arrive at the last highway/petrol stop of Switzerland, 9 km before the Italian border, called Coldrerio.
Italian Petrol is also very expensive, about LIT 1750 per liter, so fill up before crossing the border. It should be about 6 am at this point and you can stop, relax and have breakfast (there is a McDonalds hamburger restaurant here).
From here taking the highway to Milano and then Bologna you arrive at Ancona 3 to 4 hours later.
Travel agency offers ticketing and reservations on many companies crossing the Adriatic on the way to Ellada.
From Belgium, count about 14 hours to Ancona and another 21 hours to Greece. It is preferable to get off at Igoumenitsa and thus take a look at mountainous Epirus on the way to your destination. The ferries also go to Patras for the same fare as to Igoumenitsa.
The price of petrol (Gasoline) in Greece is about DRA 210 per liter(BF 27), unleaded or about DRA 200 for Diesel (BF25).