In the past I've always reserved a rental car from the US before my trip. It seems I read the price can be significantly cheaper booking it ahead from the US.
In September I'm planning at a loop in France that includes Paris, Rouen, Normandy (Bayeux), Mont St. Michel, Brittany (still deciding), Loire Valley, Chartes, and back to Paris - or (now considering) opposite order.
The transportation is:
1. Paris to Rouen, and Loire Valley through Chartes back to Paris by train.
2. Normandy and Brittany by car.
3. Loire by bike.
The biggest question is how much time I'll spend in Loire on the bike? I'll probably see a handful of chateaux, an abbey, and stay at either hostels or air bnb. I don't want to make it a race, but I'd also rather add days to Brittany if I move through the itinerary quickly.
Which makes me wonder whether it may be best to visit Loire first and have no fixed reservation for a car to visit Brittany and Normandy before returning it in Rouen, then taking a train back into Paris? What if I reserve a car but want to pick it up a day or two in advance of my reserved time?
I avoid fixed itineraries, so any guidance that allows me to stay flexible is much appreciated.