you dont earth the tank mate!! to check if the gauge is working ok you get the wire going to the gauge and earth that off and then the gauge should shoot up then come back down when released. Its gota be something to do with how its wired, i converted the gfs car to efi and the fuel gague still works fine with the old mfi clocks and efi clocks!! the temp gauge doesnt work properly as it never seems to get to temp but im hoping thats due to a dodgy stat staying open. Ive got the rev counter problem too thou, even with the efi clocks it doesnt register properly so ive gotta get another set of efi clocks too see if its that or not!!

Check to make sure the sender wire goes from/too the tank/sender ok and you have continuty going to the gague from it too with a multimeter!!