Fuel gauge check

Before I buy it I thought I would try the ground trick on the hose from the tank. I grounded it out to the exhaust pipe and checked the gauge...it went to 1/4 tank, yeehaw It works. So I ran the wire into the trunk and grounded it to the rear seat support. I checked the gauge to make sure it works and it just move a hair??????I moved it to as different spot and still only move a hair, getting POed I ran it to the exhaust again and it only moved a hair...what gives..
AL