Metric. Of course, all the units are metric - kilometers per hour (km/h, sometimes "clicks"), kilometers (km, sometimes "clicks"), and meters (m). Speed limits are posted in km/h, usually a multiple of ten. The gauges in the car are in the same units, so conversion is not required, but as an American, I like to think in terms of miles, so I take the value in km and drop the last digit, then multiple by six and round up to a multiple of five. For example, 70 km/h becomes 7 * 6, giving 42 and rounding up to 45; 100 km/h becomes 10 * 6, giving 60 mph.
Roundabouts. In Boston, these are called rotaries but we call them roundabouts in the Pacific Northwest. They are beloved structures in France (outside Paris) to the point that traffic lights are relatively rare and there are very few STOP signs. On various errands, I would barely touch my brakes while covering 35 km because everything was a rotary rather than a STOP sign or traffic light. There are two key tricks for rotaries - entry and exit.
Rotary entry is a merge process. You wan to avoid stopping; instead, adjust your entry speed so that you fit into traffic. In rush hour, this may not be possible and you will stop to wait for an entry, but mostly you will slow a bit to merge into the flowing traffic. That is it - other drivers expect it and will be surprised (unpleasantly) should you stop.
Rotary exit can be confusing, so stay alert. The first thing is to know which exit you want. Larger rotaries will have five or more roads meeting, so you may want an exit other than a right angle (right, straight, left). Some rotaries also have blocked exits (these do not count) or exits for special purposes like maintenance or parking lots (especially park-and-ride lots), and these do count. When I say "count", your GPS (or navigator) should tell you which exit number you want. For example, "second exit" is often the equivalent of straight through. In another style, my wife and I would talk about "virtual left" meaning to take the exit around 270 degrees, which could be more or fewer than the third exit.
While many rotaries are effectively a single lane, there are larger rotaries with two entrance, travel, and exit lanes and people will use all the lanes. When you enter a rotary in a two-lane entry, keep to your lane: right lane to the outer lane and left land to the center lane. Be alert for someone traveling on your left who wants to exit - let them in. It helps to signal your intentions - signal left to stay in the rotary and signal right when you are going to take the next exit.
Tolls. The major roads, Routes National or N-number roads, will likely have tolls and some smaller roads may, too, as well as larger bridges. France has an automated toll system and your rental car my have a transponder; check with the agent when you pick up the car. Cars with transponders can use the lanes marked for "e" or "EZ", typically to the left side of the block of toll booths. If you want to pay in cash, the marked lanes are typically to the right. The lanes in the middle are for payment by credit cards, and these allow "contactless" (aka "tap") if your card is capable. Sometimes there are special lanes to the far right for trucks, so stay out of their way. My experience is that the contactless lanes happily accept Apple Pay and Google Wallet, either from a phone or from a smartwatch.
Toll tickets. As you enter a Route National, you will probably pick up a ticket at the booth. As you exit the Route National, you will insert the ticket, then present payment (e.g., contactless card). In another style, you will simply pay a fixed toll at a toll booth, but there will still be EZ, card, and cash lanes.
Speed limits. Speed limits on the N-number roads tend to be 130 km/h, but this varies and can be as low as 100-110 km/h, and the exit speeds are closer to 70 km/h. Speed limits on mid-sized roads are usually around 100 km/h and speed limits on the smaller roads are 80-90 km/h, varying. As the mid and smaller roads enter towns, the speed limits will drop quickly to about 70 km/h and then down to 50 km/h. These are typical speed limits and you should always check the posted speeds.
Speeding tickets. Many towns are equipped with traffic cameras that will record your license plate number and speed. If you are speeding, the Republique Francaise will send you an Avis de Contravention - a speeding ticket. The report (ticket) will tell you when and where the contravention happened and what the infraction was, such as "exces de vitesse" for speeding, a parking ticket, etc. This will cost you a fine, conveniently payable by credit card. The rental car company will also send you a letter explaining that they were contacted by the government to ask for information about the registered driver(s). The rental car will charge you for this contact using the payment information on record (fees vary, but expect euro 40-50). There are many variables to determine the cost of a speeding ticket, but expect euro 45-180, depending on how quickly you pay and how fast you were going.
So there you have some thoughts on driving in France.