To use the Swiss motorway, you need to buy a "vignette" every year which is actually valid for 14 months (for example, the 2015 vignette can be used from December 1st 2014 until end of January 2016).
It costs 40CHF, and it is much better to take it even just for one day, they are specially checking foreigners cars (the fine is 200CHF).
You can buy it at the Bardonnex toll but it can be hectic. Or at the Automobile Club du Mont Blanc (between Bonlieu and Préfecture in Annecy) for 40€, they can also mail to you.
AUTOMOBILE CLUB DU MONT-BLANC - 15 rue de la Préfecture - 74000 ANNECY
Tel : 04 50 45 09 1204 50 45 09 12 - contact@acmb74.org
The Liane motorway (Annecy/Geneva) can also sell it to you in the mornings in December/January at the Allonzier stop area, it is convenient and they also give you a bag of small presents. Available all year on the motorway stop "La Ripaille" (Aix les Bains/Annecy).
Be careful, in Switzerland, the motorway signs are green and the road one is blue (opposite than French).
The average speed limit is 120km/h (from Bardonnex toll to the airport it is 100km/h). Be aware that they do have a lot of speed cameras.
Just before reaching Bardonnex toll (the most important one coming from French motorway) and leaving France, you will have a speed camera (50km/h), I call it the "slot machine", it is one of the most "successful" speed camera in France. But not a good way to remember leaving our country!