I had originally reported this to the Montana Beta group thinking it was a device issue. Now I think it's more to do with Basecamp or a combination of the two.
When navigating with the Nuvi dashboard active you should see the map data current speed limit above the three bar menu pick. I have found that if the Montana is navigating a route created in Basecamp this speed limit is not shown. If the Montana is navigating a route created on the device or is navigating directly to a waypoint or POI item the speed limit is shown. It seems that there is something in the Basecamp route file that is preventing the display.
There is another message board (advrider.com) for motorcycle riders that has a Montana thread mentioning the same issue so it's not just me.
Basecamp is 4.0.2
Montana is 4.60