Hi Dimitri,
I've opened up speedometers in the past and re-positioned the needle to 'correct' the reading. There's usually a little tension on the needle shaft hairspring when the needle is at zero.
If you want to try this, I'd suggest (going by the figures you've given) that, having opened up the speedo sufficiently to get at the needle, you rotate it to 70 km/h and either hold the needle shaft below the needle using mini-needle nose pliers or hold the drum behind the gauge face (i.e. the plate that has the speed numbers on it) while at this speed reading, then pull off the needle and refit it to align with say 55 km/h mark. It might be a good idea to just connect the still-opened-up speedo to the speedo cable and take the car for a run to check your adjustment before re-assembling the speedometer. The reason I suggest 55 km/h is that Fiat speedo's afaik always overread, the faster you go the more optimistically they read - if you set it to be exactly correct at 50km/h, it may under-read at lower speeds which may result in a fine for speeding in lower speed zones e.g. in town.
Note :- Sometimes the needle hairspring mounting position can be adjusted (like on a watch), should this be the case on your speedo, try this adjustment before re-positioning the speedo needle.
Another thing to check for is that no oil/grease has gotten into the speedo from e.g. over-lubeing the speedo cable, this can upset speedo readings also.
There are some U.K. companies who can re-calibrate speedometers etc., they often advertise at the back of the British classic car magazines.(Idk about in Greece)
Be gentle with the speedo innards!
P.S. To:- 'The Panda Nut', I doubt the above technique (bodge
) can be used on modern speedometers which are electronic afaik?
Al.