If the petrol guage shows empty after you fill you car up with petrol then I know exactly what it is as seems to be very common in EA's. The EA fuel guage works off some dodgy thing where the lower the voltage going to the guage the higher the petrol reading shows.
IF you actually disconnect the wiring to the guage completely the guage will go all the way up to the top... but because it has some overload thing that stops the needle hitting the top of the display thing the needle goes to the top, then drops down to the bottom because the signal is showing so much petrol and the needle can't go that high up.
You can see if this is your problem by watching the fuel guage carefully, put the key in the ignition and turn it to the start position and if the fuel guage goes all the way to the top, then drops to the bottom again. I've seen this problem in my car and if it's sort of the same thing you are having it's pretty simple to fix. You need to decrease the amount of resistance that is occuring in the guage sender to fuel guage circuit. The high resistance I found occurs at the fuel pump connector, because of corrosion. Undo the screws holding the cover over the fuel tank and remove the cover. Then clean the plug connections using some sand paper and then some metho. Fixed the problem for me, which is a good thing because running out of petrol all the time is embarrassing!