Reset The Trip Computer
I never fill up until the dashboard warning message can't cleared. I reset my trip computer at every refuel and always fill the tank full, that way i am always confident how far i can travel. The warning light comes on when the car believes it has 50miles left.
I pushed this once in France heading towards Luxembourg to 90 miles, going 55mph to maximise the cheap diesel in Luxembourg, got over 58L in a tank vauxhall says is 55L.
When i have filled up as soon as the warning light comes on I only get around 48/49 L in the tank. By my reckoning there is usually around 8/9L (possibly more)(but i don't want to run to empty)when the light comes on. Efficient driving on a motorway can get over 10miles to the litre.