I got my garmin nuvi to show me shell stations on my route so i knew how long to go for each fill up! Switzerland was the most expensive but no idea now with the euro being buggared!
The good thing about the service station signs in France is that they tell you how far they are - the next 4 or 5 services - and what company is running the petrol station i.e. shell or whatever. Can't go wrong!
