put a can of petrol in the boot and try it and see how many miles you get from the light coming on to the car conking out lol, then just stick some petrol from the boot in and fill her up at nearest petty station
It goes to ---- because it's only ever an estimate based on all the contributing factors. If you relied on the reading 100% even with the fuel light on you could easily run out of petrol before the reading went to zero. People would then try to claim from Renault for costs etc, hence the display going to ----.