Although others claim success in other ways, the only way I’ve found to get any Garmin to auto-route with useful reliability is to purchase Garmin’s City Navigator maps.
Problems like the one you describe have otherwise been so common for me as to render auto-routing useless.
Agreed. Although OSM is much better than it used to be, you can still get errors that send you miles out of your way outside urban areas.
In the real world, you're going to come up against this problem even with City Navigator, due to the low weighting of some minor roads, and because as a cyclist you're going to break the rules of the map from time to time. A typical situation is where you follow a residential dead-end street up to a main road, become a pedestrian for a minute, cross the road, and then resume on some other minor road on the other side.
While I like to use auto-routing to make the display more easy to read on my recumbent, my preferred approach is to supplement the route with a track which is set to be displayed, so I have something to refer to when there's reason not to trust the auto-routing. If you're deliberately going to pull a manoeuvre like the one described above, I find a waypoint reminding you what's going on is prudent.
The auto-routing will sort itself out when you re-join the plotted route further on. Which also answers this question:
And a supplementary question if I may: if I plan a route with (say) 40 routepoints, but decide to take a detour that avoids routepoints 18-22, will it start to guide me toward routepoint 23 when I close in on it, or will it insist that I go back to 18?
Yes; it will start guiding you towards 22 as you approach 22. For this reason, circular (and figure-of-8) routes are considered harmful - it will likely take a shortcut right to the end. The work-around for this is to split your ride into more than one route. When you get to the end of the first route, you select the next one. Split the route at a cafe or control and it's no big deal.
Other useful tips:
When crafting a route for autorouting, it works best if you put the routepoints in the middle of the roads you want to use, rather than near junctions. A particular issue with dual carriageways and slip-roads is that they're composed of logical one-way roads in the map, and a routepoint on the wrong 'side' will cause it to perform a detour and u-turn to join up.
Use Basecamp (or Mapsource) to plot your route against the actual map that's on the eTrex, rather than some online tool. This is the only way to ensure consistency in the maps.
In their infinite wisdom(!) Garmin have used a different routing algorithm in Mapsource/Basecamp than they have in the unit, so while the maps will be consistent, the routing decisions won't be. You'll have to sanity check the routing decisions made by the device itself. This is tedious to do in advance due to the slow scrolling/zooming, and a source of delay on the road. Your choice.
Most audaxers will tell you that auto-routing is a mug's game and you're better off following tracks by eye. They certainly have a point. Personally, I find it more worthwhile in urban areas, particularly if I'm just wanting to get somewhere and I'm not really fussed about what route it actually takes.
