Empty tank reading = faulty tank sender or break in the wiring to the tank sensor or faulty dashboard regulator (10 volts)
Full tank reading = faulty tank sender or short in the wiring to ground or faulty dashboard regulator (>12 volts)
If your temp gauge is working ok then the dash regulator is probably ok. Try grounding your petrol tank sender wire and see if the gauge goes through the roof - if it does then you've either got a bad connection on the tank sender or a faulty sender.
i had problems with my petrol gauge a while back when i was replacein a knackerd back window screen wiper moter while it was out in the car if i had my foot down on the break pedal the fuel would go rite down and as soon as i took my foot off it would go back up again . but as soon as i put the moter back in everything worked fine again