mine seems to do that aswell my solution is too only fill it up half way petrol seems to last longer!
all the different computers read outs,i.e mpg,miles to next fill up,miles driven since last,litres in tank never add up.
you know when your petrol light comes on that you can rely on that though!
can i ask though lol whats the furthest anyones ever gone on here with the miles to go blank and the fuel light on an whos all run out?i tried running mine dry today so to get to the fuel tank.drove about 45 miles,ever tried booting it for five,got back to my driveway and left it sitting thinking it was about to go,thing just wouldnt die an looked in the fuel tank an their was still petrol left!