Garmin Navigation issues

I am running a Garmin Edge 810 with OSM maps of the Eastern US. I ride brevets where two kinds of data prove to be very important.  One is elevation and the other navigation.  I want to focus on a navigation issue for now.  

On a specific planned route, the Estimated time to Next is correctly calculated by the Garmin.  BUT, Estimated time to Destination is invariably very incorrect and usually by a large time.

In talking with Garmin support they say that they cannot comment on the issue since I am using OSM maps. But I note that the Distance to Next and Time to Next is always correct.

Distance and Time to Destination is a simple calculation and could be based on a fixed average speed or the current average overall speed.  But the Time to Destination is so far off as to make me wonder what speed it is based on.

Does anyone have experience with this issue?


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