We are planning an approximately 21 day trip touring SW France and the Basque region. I am intrigued, looking at the map, that there seems to be a kind of gap in large roads southwest of Sarlat. I am hoping to do a circular route from Bordeaux to Sarlat; south from Sarlat to somewhere then further south to St.Jean de Port or Ainhoa; west to St. Jean de Luz and continuing west to Bilbao visiting the towns along the way. We'd return along the more direct route to Bordeaux. We enjoy small towns and local markets. I would welcome comments, suggestions for overnights/eating along the way and sights along the way. I truly appreciate the input. Many thanks.
If you draw a straight line from Sarlat to Saint jean-Pied de Port, you first hit a fairly hilly area (hence no roads), then a sparsely populated area between the Landes forest and the Gers.
The more interesting route would be through Toulouse. Between Sarlat and Toulouse, Rocamadour, St Cirq Lapopie, Cahors make for interesting stops. Toulouse itself is worth a day of exploration, and both Carcassonne and Albi can be visited as day trips from there.
Between Toulouse and the Basque country, Lourdes could be of interest as a stopover, otherwise if you do not wish/have time to explore the Pyrenees, you might as well drive straight through (considering a lunch stop in Pay maybe).
I recommend Rocamadour only if you can allow sufficient time to go up to the Cité Réligieuse. The historic street on the lower level is now an essentially unbroken strip of tourist cafes and souvenir shops. It was mobbed at the time of my visit in early June 2019.
As Balso mentioned, the area between Sarlat and the Spanish border is not of great interest, on the second half of the route you will cross the Landes forest via secondary roads, many of which are endless straight lines through the forest.
The itinerary he proposes is more interesting.
I complete it with this one:
https://goo.gl/maps/5pYLgign3onq7SGM7
It goes through the places already mentioned:
Rocamadour, Saint-Cirq-Lapopie then
Caussade
Bruniquel
One of the "Most beautiful villages in France" with a medieval castle. https://tinyurl.com/2p3juare
A few kilometers further:
Puycelci
Another medieval village also one of the "Most beautiful villages in France" https://tinyurl.com/2s3dc4d3
Gaillac
A charming provincial town surrounded by numerous vineyards
Albi
The capital of the Tarn and the episcopal city all in red bricks with its impressive cathedral. https://tinyurl.com/4vc9a3c9
Toulouse
The 4th largest city in France and capital of Occitanie region . https://www.toulouse-visit.com/
Since between Sarlat and Albi, there is a lot to see, you can spend the night for example in Caussade which is a pleasant little provincial town (whose specialty is the manufacture of hats), or in Cahors, a slightly larger town well known for its wine (which I am not fanatic).
And the next night (or nights) Albi worth spending a night there as well as Toulouse if you have the enough time.