fuel gauge woes

[prostreetC-10]

This is actually for my 73 truck but I'm hoping you guys might be able to help. I have ready different posts about the sending unit and how it should have different ohms for empty vs full. What I'm trying to figure out though is if there is a chance it's the gague and not the tanks? Reason why is that it's duel tanks and each one does something different. The left tank will say 1/4 full whether it's full or empty. When I switch to the right tank, it goes off the marks and over to the 3 O'clock position. Any thoughts? Thanks!