There's a couple of things to check first. Take the sender wire (the one coming from the guage) off at the tank and let it dangle in the air. Turn the key on - the guage, if working correctly, will go to "full". If it does go to full, check the wiring at the tank sender, there was a post here a few weeks ago where the wiring was reversed (the sender wire went to the ground clip and the ground wire went to the sender),
If the guage doesn't register full when you remove the sender wire, the trouble points back towards the guage. Carefully check that wire all the way back to the guage, it could be pinched and grounded somewhere. Check the wiring behind the guage, and also, the guage case has to be grounded to the dash, either by a ground wire or a good mechanical ground.
