Like most good ideas this is really simple. The speed converter sticker we show here for your windscreen is such a useful driver aid. If you're taking a road trip through France, where the speed limits are stated in kph it's a no brainer.
This this article for guidance on speeding fines and speed cameras in France
It is always advisable to check what the kph limits are in any country you will be driving in, ignorance is no excuse when it comes to getting an on-the spot speeding ticket. You should also check our useful page all about French Road Signs