According to page 59 and onwards of the
Road Traffic Handbook of 2012 (though it's only in Greek):
For trucks with trailers or rigid trucks:
Motorways: 80 km/h max.
Expressways: 70 km/h max.
Secondary roads: 70 km/h max.
It's worth noting that all trucks carrying hazardous cargo have to go slower than the limit mentioned, and the type of cargo dictates how much slower they should go.
For trucks with no trailers:
Motorways: 85 km/h max.
Expressways: 80 km/h max.
Secondary roads: 80 km/h max.
In both cases the urban limit is 50 km/h unless indicated otherwise by a sign. Also, though I think it's obvious, all trucks (and not only) are required to display a sticker at the back of their vehicle showing their maximum speed limit (fail to do so and it's a 80,00€ fine).
Now for anyone who's curious, here's a list of the fines one must pay for not following the speed limits (this applies to all vehicles):
Up to 20 km/h above the limit: 40,00€
Over 20 km/h above the limit: 100,00€
Over 30 km/h above the limit: 350,00€ and no license for 60 days
Anything below the minimum speed limit: 80,00€
(Minimum speed limits doesn't exist on all roads and if there is such a limit on a road it is displayed on signs like P-57 on page 164 of the handbook if it applies to the whole road and Π-68 on page 171 of the handbook if they vary for each lane. This is also why I put max. next to the speed limits above as they are the maximum speed limits. )
Lastly, trucks and buses are required to have devices which work like black boxes on a plane. This device records the distance driven, the number of times stopped and the time of each stop as well as the speed of the vehicle at any given moment. Failing to have the vehicle equipped with such a device, having a deactivated device or intentionally modifying the data on the device results in a 700,00€ fine and no license for 30 days.
Hopefully that should be enough (though some of it is irrelevant
).