hmmm, although I like this idea, I believe there are more important 'tracks' to implement.
1. Tour de Corse (Corsica Rally/Rally of 1000 turns)
Located on the island of Corsica
Group B used to race in this rally
Amazing scenery, elevation change, difficulty
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Second would be the Targa Florio
another very famous road race and one of the oldest sports car events, originally a lap of the island of Sicily, later the lenght was reduced to 72kms.
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Also, something more along the lines of what you were looking for, would be the Mille Miglia, which actually runs through areas like the Toscana region (featured in GT5).
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Sadly I realizing that to create a track or stretch of road, to the accuracy PD would like, both in terms of physics and graphic reality, would take huge amounts of effort, time and resources
I wonder if PD would be able to merge together the Toscana and Eiger track creator, to form some kind of long stretch of road which passes from countryside to mountainous region to simulate the Grand Tour event