Originally Posted by androidGB
I was thinking Calais > Rouen>Le Mans>Tours>Poitiers>Bordeaux>San Sebastian>Burgos>Madrid>Granada etc
But always prepared to listen to alternatives
The above route works well in my opinion. The only catch may be, if my memory serves me right, is that sat navs and perhaps signposts tempt you to cut a corner between San Sebastien and Burgos, i.e. missing the southern outskirts of Bilboa. But some of this shortcut is a single carraigeway, up and over some hills, rather than a dual carraigeway or motorway. It is easily passable, but is favoured by quite a few slow moving lorries.
Looking at autoroute now, at home, rather than on the move, and again if memory serves me well, it appears that the single carraigeway goes via Mondragon and lies in the Basque area and this might explain why the upgrading of the diagonal shortcut has not been completed (or started).
Repeat the diagonal works, but you may prefer to continue following signs till Bilbao and the A68 south towards Miranda de Ebro, and then Burgos. I have done both and my preference is for the longer way round.
If anyone can be correct this or confirm this, please do.
Best of luck.