We have found that to save money when buying fuel especially if you are using petrol. is to use the french government site " Prix de carbauants" look up towns along your route and there are plenty of fuel stations close to the autoroute where you can get your fuel at normal prices. The fuel site includes a local map showing the filling stn and prices which are very regulary updated earlier this month diesel was around euro 1.04 per Lt. it also covers belgium as well I believe.