I agree with all others that driving from Tours to Chartres to CDG on the day of your flight is "a bridge too far," even if you weren't stopping to see the cathedral. I think your plan to spend Saturday night in Chartres and drop the car there makes more sense -- if you get to Chartres early enough Saturday to do your cathedral visit that day.
An easier website to use (for planning, not for buying tickets), is this one. It shows hourly service from Chartres to Gare Montparnasse on that Sunday morning, followed by a Metro to St-Michel and transfer to the RER "B" for CDG, totaling about 2 1/2 hours. I'd allow more for possible delays within Paris. As others said, you don't want to take chances with a high-stakes flight. Also, schlepping your bags on the Metro and then RER would be a chore. Those trains could be very crowded on the Sunday of the Tour de France. Taxi between stations in Paris (Montparnasse to Nord, say) wouldn't cost too much but might take awhile because of Tour-related traffic. And this whole scenario would assume you get on the train at Chartres early enough in the morning -- probably too early to see more than the outside of the cathedral.
If you can do your cathedral-visiting on Saturday (including Malcolm Miller's tour if possible), you could take an evening train into Paris and spend your last night there. That would give you lots of time Sunday to get to CDG for your flight. Having dropped the car at Chartres, you won't have to deal with it. Staying somewhere near St-Michel or Denfert-Rochereau would put you close to the RER to CDG.
But of course going to Chartres on Saturday, early enough to see the cathedral, takes time away from whatever you were planning to do in or near Tours. These are the tradeoffs, I fear. If you expect to return to Paris in the future, consider Chartres as an easy day trip from there and save it for then. There are fast trains that Sunday from Tours to Gare Montparnasse, followed by the Metro-RER or taxi-RER routes described above.
If it were my trip, I'd go to Chartres Saturday and Paris Saturday evening. But I don't know what I'd be missing Saturday in or around Tours. Hope that helps.