Forget carputers.....!
Anyway, here is an off road GPS I just found, AvMap:
http://www.avmap.us/products/terr/geosa ... troduction
You can import maps from a variety of sources and convert them, click through all the tabs and you will see.
Click on the "4x4" functions on the picture of the menu, it shows a video of what it can do.
Or view video here:
https://www.facebook.com/video/video.ph ... 5968359069
I haven't bought one, just came across it. These also work with APRS if you are a ham, which is why I was checking them out.
Here is a picture from their Europe product, I assume it looks pretty similar when you upload your own topo map: