Speed limit listings are just like map data (streets, names, etc.) - there are two kinds.: static and active. Now I don't know how the new built in Nav system handles speed limits but I strongly suspect they are static just like the maps. That means there has to be a map upgrade to change the speed limits and hopefully you can imagine how long that will take. It's also probably not up to Ram as I'd bet they buy the map data from someone else.
Now, if it's active (like Waze), maybe through Sirius Travel, it can be updated much more quickly but you'd still need to find out who Ram info provider is and get them to make the change and then wait for it to get to your truck. Technically faster but still not something to wait on.
In the end, this is just how nav systems work. There will never be a perfect, 100% accurate map with speed limits because as soon as someone published one, town xyz changes a speed limit and it's wrong again. I've not looked but you know there is language in the license agreement about "always use actual speed limit signs" instead of the nav speed limit just like "always watch where you are driving" instead of blindly trusting the nav by turning right - into a lake.