A digi tacho records everything, from speed, how hard you brake, working time, driving time, how fast you accelerate, your rest, ANY movement.
A drivers card can keep a MINIMUM of 28 days of data, but many can show date from 8-10 month back.
It all depends on how many time you change mode, driving from A-B is only one thing, putting it on other work is the next one.
The drivers card doesn’t record speed, only driving, work, rest and manual entries, driving without card and other errors, if not reset before you enter your card in the tacho head.
The tacho head holds at least 3 month of data, but if you download you realise it shows from up more than a year ago.
Long distance trucks have mostimes 2-3 years date on them as they relative little else as driving.
The tacho records in 1hz (stoneridge) or 2hz (VDO, Kienzle) so it measures everything in 1 second (stoneridge) or half a second (others)
That’s why every movement is visible, even just a touch forward.
Regarding overspeeding, this can be a customer setting, some customers want is set on 80, 85 etc. Standard is 90km/h.
From experience the Germans will fine you if you have a substantial overrun of the speed, letting it fall down the mountains is not advisable.
The British police / Vosa seems to have a more relaxed vision on this.
However as with everything, if you get a major accident, this could work very much against you, if your downloads shows regular overspeeds.
Another thing what goes a lot wrong with digi tacho is rest.
Drivers who work at the stopwatch to take exactly 45 mins, you could stop 13:15 hrs and 59 seconds, you pull away at 14:00hrs and 2 seconds, and you will find out that you haven’t taken a complete rest, and have to take another half hour to reset the tacho.
