Nice town south of Paris

As others have pointed out, the Peripherique, or Autoroute encircling Paris would likely have heavy traffic around 19:30-20:00, when you would likely be driving, and it would probably be getting dark at that time in mid-September.
At 08:19, there is a TGV from CDG to St Pierre-des-Corps ( a suburb of Tours), where you could transfer to a regional train ( TER), and arrive in Tours at 10:09. You would have to get a hotel at CDG for the night, but you'd lose very little time for touring the Loire Valley, and arrive in Tours relaxed and fresh, rather than frazzled from having driven from CDG. You could the rent a car from the station in Tours, and be in someplace like Amboise in just a few minutes.

Ok. Got it. I thought it was the A-roads - the motorways.
I guess I could trust Google maps suggested route then. What is a bit confusing it getting out of the airport. If you for instance compare it with Malaga they will display the whole route from the car company to your destination. At CDG the displayed route starts just before the motorway! The airport area seems to be extremely complex if you start to examine it in detail.... But it will be allright.
We will look at Rambouillet or Melun on the trip back. Our flight leaves about 2 pm on Saturday and an hours drive from the airport should be ok.

Basie: Is it heavy traffic in both directions? Google suggests D212 from the airport and then the N2 for a couple of kilometers and then turning off to the A104 to the south. In my mind the A104 should not be effected to much from the traffick to and from Paris. Even though perhaps the N2 will. In that case it seems to be possible to avoid that road by driving on on the D212 and enter A104 further to south.

Listen to the people telling you that a car to or from CDG is a Bad idea.
Major anxiety and road complications... Think JFK on steroids.

Trains are excellent and not expensive...ESP 2nd class purchased in advance
TGV service can start at CDG in basement station..elevator down!

See details of train booking at
Man in seat 61 website

France has 8000 medieval villages
Some close to Paris.. Like Chartres...a Very good one...

In avril we found Bourges hectic and not worth it...a faceless modern city..with only a few old things
While Chartres is whole medieval town

Loire is just few mega palaces of rich 500 yrs ago and tourist trade towns now

If you are only in France for 3 days, why spend it on difficult roads?

Stay in 2000 yr old Paris, w day train trip to Chartres.?.Read Adams on Chartres..

Be smart

I hope that someone who lives in Paris can comment on expected traffic from CDG at that time of day! My belief is that your planning has been optimistic, and that you might find yourselves stuck in heavy traffic, and not making much progress towards your goal even at 21h00.
My only experience was with returning a rental car to Orly Airport, where I'd agreed to return the car by 20h00, and encountered very heavy traffic going towards Paris from the Loire Valley. I was surprised because I'd thought that at that time in the early evening, traffic would be very heavy from Paris southbound, and I was getting very worried about teaching Orly in time!

I hope that someone who lives in Paris can comment on expected traffic from CDG at that time of day!

That would be me.
First, driving around CDG isn't nearly as bad as some people (well, one person really) makes it out to be. Yes, it's a big airport, with lots of flyovers etc. So usually when people think about leaving from Paris to take public transport to CDG to pick up a car there (in order to avoid Paris traffic) i don;t think that makes any sens, as dealing with CDG is as much of a hassle as dealing with Paris proper.

But of course, 1000s of people do it every day and you wouldn't be the first one to leave the airport in a rental car so there are signs etc and you should be fine.

Yes, traffic can be an issue. It depends on the day of the week (Fridays are worst) but you can expect traffic to flow out of Paris between 6 and 8 pm. The Perif can be chock a block.
You can check traffic at Google Maps:
https://www.google.com/maps/@48.8588589,2.3470599,12z/data=!5m1!1e1

Google driving times do not account for traffic.

the A roads are highways (not just the Perif). (They also have an E number which is the European highway number)Avoiding them does not make for a faster journey i think. Just avoid the Perif: eg from CDG take the E15/A86 and then on to the A6 or A11, depending on where you're going.

Frankly, taking a train from CDG (and allowing enough time for a delayed flight) is not going to be faster. And spending the night in Paris if you're not interested in Paris is not a solution either.

I'd also like to refute the notion that 'Loire is just few mega palaces of rich 500 yrs ago and tourist trade towns now'

As a matter of fact i just spent 3 days around Blois last weekend. I've been a few times before. There are some large castles attracting lots of tourists but there are also plenty of small, intimate chateaux, villages and landscapes that are very much 'off te beaten track'. You'll have no problem finding them if you look for them.

See my two articles here:
http://www.minorsights.com/2015/07/france-cycling-loire-valley-around-blois.html
http://www.minorsights.com/2014/07/france-chateau-de-villesavin-family.html

Capptim: I admire your passion but you are not really giving us any advices. We are going to Loire. We want to get away from Paris, not into Paris. We will have a car. There are no Sancerre vineyards in Paris.
Train will be 200 euros for two persons. Plus car 150 Euros. I am sure we will arrive earlier by car than by train.

Traffic is still a concern though. Spent some time yesterday studying maps (google and Michelin) trying to find my way out of CDG. The planners must have been on LSD. Must look great from air though... But I guess Avis should be able to show us the way out. From my point of view one should be able to turn east from terminal 1 and drive at rue Paris north of terminal 2, and via Rue de New York and furhter to D212... Google and Via Michelin however suggest we go straight to E15.
I have small experience of France (first trip me and my wife made as a young couple in 1984 plus a couple of business trips) - but my impression is that everything seems chaotic at first, but for somehow it works out finely..
Thanks for the links and advices Hello_Bamboo - just the kind of information we are looking for.
I am listening to what you are saying about the traffic and perhaps we are a bit optimistic. We are now looking for an alternative for the first night. My wife just came up with Montargis. Any experience?
Regarding traffic - there is in fact a traffic option on Google.

I don't know where you're coming up with train prices but picking a random Tuesday in mid September (I chose Sept. 15th) I see a few one way fares for 29.70€ from Gare Montparnasse to Tours and the journey takes 1 hour 14 minutes. Use www.capitainetrain.com to check schedules and prices and to buy tickets. Tickets are cheapest when purchased up to 3 months in advance. The TGV train will always be faster than the car between Paris and the Loire.

However, I won't go on about trains since you seem to have made up your mind about using the car. First I would say do not use google maps as it always underestimates drive times. Use www.viamichelin.com, which also has far more options to choose from and when you zoom in on the map you get far more detail since it will show you the same thing as if you were using the 1:150,000 scale Michelin départements maps. The drive times don't consider stops but are reliable otherwise. The drive times also are current so perhaps googlemaps might be useful at least for getting traffic impact estimates.

In regards to driving from CDG to a destination south of Paris and departing CDG at 7:30 on a weekday you are likely going to encounter heavy traffic. I would forget Fontainebleau, Melun and Montargis as these are in the wrong direction if your destination is the heart of the Loire, meaning from Blois westwards. Rambouillet would be a good choice but you better do some restaurant research because you likely won't get seated after 9PM or 9:30 on a weekday, depending on the restaurant. If you are in the car and leaving CDG at 7:30 and don't hit bad traffic then you should be alright to make a restaurant in Rambouillet but any delay in leaving the airport or any heavy traffic will impact your ability to make a restaurant. Chartres would also be a good destination and would be a better place to enjoy strolling around in the evening as there is much more to it than Rambouillet. More restaurant/food options in Chartres as well. Look at the tourist office websites for both places to learn about accommodation/restaurant options:

http://www.chartres-tourisme.com/en

http://www.rambouillet-tourism.com/en/the-official-website-for-rambouillet-tourist-office.html?no_cache=1

d Accord......merci

CDG is a terrible place to get a car ..

Take Air France bus from cdg to Gare Montparnasse

Then hourly 19 euro train to Chartres ..

Enjoy one of theBest medieval towns in France...great restaurants and wine

Most memorable cathedral on top of hill town..

Good reasonable hotels

Then get car
Tour
Return it there

Avoid chaotic Paris roads altogether

Remember the major crop of.
France is wine

There are vineyards everywhere

Even one in Paris !

Mon dieu