One of my gas tanks is reading incorrectly

[99-LS1-SS]

I've got a 1984 K10 with a 305 and dual gas tanks. When I am switched to the right hand tank the gauge reads correctly. When I'm on the left hand tank the gauge reads way past the full mark(approximately at the 3:00 if you were to look at the gauge like a clock). It has recently started doing this. Has this happened to anyone else? Where is a good place to start for fixing it?

Thanks.