To get the best rates and save up to 50%, one should always make rental car arrangements before coming to France. I have used Auto Europe, Avis, and Holiday Auto and Kemwel and been happy with all four. I have also used Budget, but wasn't impressed. They will usually match the lowest rates of one another if you tell them that you've found a great rate elsewhere.
For rental cars, if you're into Citroens, there used to be a rental car company named Citer which used to offer nothing but Citroens, but it's now part of Enterprise. If you like minivans, then try the Renault Espace. The Peugeot 508 is about the size of a BMW 5 series, comfortable and big enough for four or five passengers. Of course, you can also get many great German, British, Italian, Spanish, Japanese, Korean and other cars. Make sure you have insurance while driving in France. Many credit cards offer coverage. You may also be able to save money by bringing your GPS from home, but preloading it with a map of France or of Europe. Garmin, for instance, offers this service for a fee.