It seems the French are no good at such calculations. My Renault's fuel guage is very non linear. The second half of the tank is used twice as fast as the first half according to the guage. As for the digital "miles left before running out" reading, it seems to think I can get about 90mpg from a petrol engine. But luckily the average speed since you last pressed the button guage is very very accurate, so I use it for cheating the average speed cameras. People just don't get why I can go 100 in a 60.