The Massif Central region around Millau has dozens (hundreds) of brilliant roads, we do that region a lot on the bike in the summer. Get off the Auto route at Claremont and head in to the Volcans and work your way down from there. You can do the whole journey from Calais without going near an auto route, but France is so big it can take you far too long, depending how much time you have. The Gorge du Tarn is nice, but the road isn't the best IMHO, there are better. Infact I headed up out of the gorge up onto the plateau which was better, the gorge road can be quite busy. Just get a large scale map and head for the twisties, the topography of the land means that good roads go in almost every direction
Stu
Also have a look at BestBikingRoads.com for some tips, you may be able to join them up into a route
Edited by M30, 04 June 2013 - 05:18 PM.