I struggled with this same question for a while. I'll add a little more detail to the previous response.
The issue is the different map sets are in different layers. The most detailed layer shows by default, and that's usually the City Navigator. All the other layers are hidden below. To turn off the City Navigator, do the following.
1. Go to the Map Setup page (easiest way is show the map page, hit Menu, then choose Setup Map)
2. Scroll over to select the Information Icon (the "i")
3. Hit Menu button again
4. Choose Hide the City Navigator Map
That should do it. At step 4 above, you have lots of choices. You can toggle the different map sets on and off (Hide and Show). I find it simplest to Show None, then Show US Topo when I'm offroad. Then when I go back to the streets, I choose Show All.
Hope this helps.