Here is a way to keep the speed limit sign on the HUD (temporarily) while driving, even after booting has completed. Not practical solution, but does demonstrate that it still works, but is being turned off by something.
1. Before doing this you have to make sure of a couple of things: a) make sure that when you put car in reverse, that the surround view cameras come up on the display (i.e. when parking). b) Make sure the car has been off long enough that when you first start it the speed limit sign will appear in the HUD. Overnight works, but I think the required time is much shorter than overnight.
2. Start the car and immediately put it in reverse. - Before the hUD starts up. The surround cameras should appear on the display.
3. Wait a few seconds for the boot process to complete - 30 seconds or so - not critical. The HUD should be displayed with the speed limit sign showing (but no speed limit yet - all dashes)
4. Put the car in "drive". Then move forward slowly but do not drive faster than the speed that turns off the surround cameras ( I know 6mph or less works).
5. Drive (6mph or less) to a street that has posted speed limits. The HUD speed limit sign will now display the posted speed limit.
6. As long as you drive 6mph or less, the speed limit sign sill stay on the HUD and will function.
7. Once you speed up and the cameras turn off, the screen asking you to confirm which driver is driving will appear and the speed limit sign will disappear from the HUD.
Again this just shows the HUD speed limit sign is functioning, but something is just not allowing it to remain on the HUD.
It is acting like in setup there is an option to display the speed limit in the HUD, but it is turned off. No such option exists, but that is what it is acting like because it disappears as soon as the "confirm who's driving screen appears".
This is the exact situation I ran into with the NAV "avoid freeways" option when I first did the upgrade. Before I upgraded, I had the "avoid freeways" option turned on. After upgrading, the "avoid freeways" option was no longer in setup options, but any route I did would display a message that it was routing & "avoiding freeways". There was no way to disable it because the option was gone from the setup menus.
The way I fixed it was by doing a full reset (setup--general--reset--reset all). After that the "avoid freeways" was no longer active.
This situation was like the reverse of what is happening with the speed limit sign in the HUD.