Here's what my experiment just showed going for a short drive.
From my home speed limit showed 2 dashes (it's not a posted zone).  Onto next street it starts in a 25 zone which displayed properly.  As it increased to 35 then 40 zone the display increased properly.  I made a U-turn, stopped (in the 40 zone) & turned off engine, and pulled the Nav SD card.  Upon restart, it showed 40.  I drove back & when I hit the 35 then 25 zones, the display never changed from 40.  U-turn again in the 25, then 35, 40 - no change; showed 40 the whole time.  Stopped in the same location, reinstalled the Nav SD card, restarted and drove back.  Display now matched the posted zones (40, 35, then 25) then - - when I turned back on my street.
So....   Seems to pull the info. from the Nav.