I have to agree with Sports850 on this. The ONLY way to really know is to drive until you run out and note where the gauge was at the time. (make sure you have spare fuel with you). As long as your gauge is COLNSISTENT, you'll be able to tell to a fair degree of accuracy.
When the gauge on my van reaches empty, I still have around 100-120km range.
The needle goes well below E before running out, but if the needle starts to rise again (must be a dicky sender) then I have about 20km max left. I ALWAYS carry a spare 10lt in the Mini, and I could write a book about where I have run out of fuel. As long as I've got spare, it is never a drama - well, almost never. There was this one time.....
Cheers, Watto.
