I merely followed the information given by VDO, whom I gather are one of the world's leading suppliers of tachographs.
I quote:
How much memory space does the DTCO have?
With average use the activities of approximately 28 working days can be stored on the driver card. The capacity of the DTCO mass memory is enough to record the activities of approximately 365 days.
Speed recording is limited to 24 driving hours and is only recorded in the mass memory of the "digital tachograph".
I do accept that using 'driving hours' then assuming an 8 hour shift, a maximum of 3 calendar days of speed information can be stored. I also note that the EU Directive controlling all this makes absolutely no mention anywhere of speed control. It is merely looking at working hours, which is fair enough but has no relevance to using a tachograph as a speed defender, methinks.
So, as a layman it would appear to me that a truck driver could possibly drive at whatever speed his vehicle could achieve, and only be penalised if he/she was caught by an external speed detection device. Even downloading as you state, weekly, is not covering the whole information. I also note from a number of sources that introduction of the digital tachographs is a result of 'inaccuracies' in the current analogue instruments. I am not able to fully ascertain what that actually means, but I assume that the checks so far have not revealed accuracy of data that would like to be seen, or why change?
I am sure many Companies and drivers follow the rules. After all, if they had no doubts they wouldn't feel the need to write on the back of their vehioles 'driven carefully, if not call....'