SK53
(SK53)
2
You certainly need to use the --route option of mkgmap to have any hope of a routable map. Its not documented on the main wiki page but there are notes here http://wiki.openstreetmap.org/wiki/Mkgmap/routing
Its worth downloading a pre-built map with routing in just to test that the process of getting the maps on your Nuvi works OK. (See http://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin/Download, I think Ecuador Tour and MTB map do routable files)
Using GroundTruth as a wrapper for the process may also may make life a bit simpler.
HTH,
SK53