There definitely seems to be differences in peoples readings, though that could just be people not going all the way. As said previously, the only way to tell is by how much you can put into the tank, or driving until you run out. Just worked out that I put in 67.85 litres, so had loads of juice left. Trouble now is, I will probably end up risking it in the future and end up stranded at the side of the road miles from a petrol station