Hi cosminache123,
if I understood you right, you made a selfmade electronic for the speedometer, situated in the place where the original speedometre cable is plugged in the gearbox.
So what you need ist the "Wegstreckenzahl", which can be read directly on the instrument cluster of the original speedometer.
There you find written e.g. K=1125. This means the speedometer cable takes 1125 revolutions per kilometer.
But keep in mind , if you changed the gearbox or the tire-dimension also the "Wegstreckenzahl" will not be the same anymore.
So an information what gearbox (F10, F16 or so) and original and future tire dimension, would help to solve your problem.
Greatings to Romania
andy67