In Belgium 120Kph on autoroutes, 80(90?)Kph outside towns, 50Kph in towns - well signed as you cross the border.
In France, 50Kph in towns (starts at the town name sign, ends at the town exit sign - town name with bar through it

90Kph on other roads (80Kph when wet

130Kph on autoroutes (110Kph when wet.)
Note that when you leave an autoroute in France the speed limit signs on the slip road are mandatory - the fuzz sometimes put a radar between them. Also, given the 'priorite a droit' rule - which still applies in many places - the entry slip road on an autoroute has a 'give way' sign at the end and the French will stop at the end of the slip road if they have not seen a gap in the traffic to join the lane - the idea of accelerating to slide into a gap doesn't seem to occur to them! Read up carefully on Priorite a droite as, until you are used to it, it can be potentially VERY dangerous. A word or two of advice. The French style of driving is IAI - intimitadory, aggressive, and impatient. If you are not travelling at the speed limit within milliseconds of a traffic light turning green you will be treated as a pariha. If you travel at less than the speed limit by even a needle-width you will be honked, flashed, and gesticulated at before they dangerously overtake.Within the 'impatient' category, they will follow you at a car length even at 90Kph. If you cannot see a car behind you through your extended mirrors don't assume there isn't one there! When you turn right at any traffic light almost anywhere in Europe be prepared to find pedestrians crossing the road. Curiously however the one thing the French rarely do is to cross a single white line in the middle of the road, even if the road is free of all other traffic, visually unobstructed, and they are following a slow moving tractor. Road signing is also unusual. Apart from exit roads on autoroutes they never use road numbers, only place names. Similarly if you ask someone how far it is to XYZ they will tell you how long it takes - use this to work out the distance assuming travelling at the speed limit throughout. Your maps or satnav may tell you to follow a French Route National (the equivalent of our A roads) and they are usually marked in red on the top of signposts a Nxx. However over the last few years many of the RN roads have been handed over to Departments (counties/boroughs) and so become RD and are signed as D roads. Most of these will have a D followed by a number in excess of 900. Note that as all Department numbering is considered local, you will likely have the same numbered road - e.g. D21 - in two adjacent Departments but they bear no relation to one another in any way. D900+ roads ahowever do have commonality.
In Belgium you will often find that a sign points you to somewhere, but at the next junction that place is not signed. Always assume to keep straight on (or as near as possible to it) if the signage is missing.
The A16 from Boulogne East to the Belgian border is free throughout as are all Belgian motorways. If you are travelling via Liege the best route is to take the autoroute towards Brussels, then turn off at Gent towards Antwerp and cross to Liege. Unless you have done it before the alternative route - the Brussels Ring - is a terrifying place, nearly as bad as the Peripherique in Paris! French motorways are always free unless the word 'peage' is on the sign pointing to it.
Two final points: Do not turn your nose up at McDonalds as you will always get a cup of decent coffee (ask for Americaine or you will get Espresso) at a fair price and a clean bog, plus they have free and unlimited wi-fi and they don't mind if you plug your laptop into their mains. In Belgium (and increasingly in France) you will have difficulty finding a manned fuel station outside office hours. Worry ye not, all French automated stations and many Belgian will take a UK credit card - and many of them give you the option of or will automatically speak to you in English. If you don't already have one get a UK credit card that doesn't impose transaction charges overseas - Nationwide Visa is very popular in that line. As of last Friday diesel in French supermarkets was around €1.29/litre.
Good luck, and enjoy.......
Woody
N Yorks
Note that it is an offence in France to have a radar detector let alone use it, and in Belgium it is an offence to use cruise control on a motorway - although how the latter could be policed baffles me.