I downloaded the the danish OSM file for Garmin from http://wiki.openstreetmap.org/wiki/WikiProject_Denmark/Garmin, and I’m mostly impressed with the result. However, when my GPS unit calculates a route to my brothers house, it takes a different route than with City Navigator. I believe the CN one is shorter / faster. I have checked the CN route on OSM, and not found any unconnected nodes (and the roads aren’t missing either).
Does the OSM map converter use different road types than City Navigator? That would give different expected speeds and thus different expected ETAs for the two routes - which could be enough to change which one is faster. Would it just be a matter of letting the unit learn the speeds again? And then what happens if/when I switch back to CN?
Garmin maps created using mkgmap from OSM data using the OSM highway type to assign a road class and road speed. Depending on which options are used when the map is created, mkgmap can also downgrade a road if it contains traffic signals or road-calming. This is probably why you’re seeing differences in route-planning between CN and OSM.
Yes, it is possible that certain roads get different road type id’s in different datasets. Determining the road type isn’t always exact science, so a secondary road in OSM might not be a secondary road in CN.
After the trip, the usual route is suggested, so I guess the unit just needed to learn the speeds again.
A couple of sub-optimal things, though… The roads are drawn much wider than with City Navigator, and when driving in the city, the display (60 CSx) gets very crowded. Also the area polygons, like residential, commercial, industrial get shown, and the colors do not change to dark colors when the unit switches to night mode. So instead of white roads on a dark background, I get white roads on a brightly lit background. Basically, everything looked white.