Hi etienne2000,
it seems to me that the reason for "wrong" routing ar speed limits which are not mapped in OpenStreetMap. So any routing programm will calculate wrong way if you choose fastest. You can check for example here:
or here
http://www.itoworld.com/product/data/ito_map/main?view=35
if the roads you use have maxspeed-information. Feel free to add a maxspeed-tag on the roads in OSM if you know these informations.
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