There's a pot on the circuit board. Little thing with a wheel on it. Damn hard to adjust because you can't change it with the cluster in. Gotta pull it out and pull it apart. Now I remember; on my '91 I drilled a hole in the front of the cluster so I could use a screw driver to adjust with it installed. On my Rabbit gti cluster, I desoldered the pot and extended it outside the cluster.
Put the gas cluster back in and see how the fuel gauge works with it.Might just be the gauge itself.