Gas gauge inaccurate. bad Sending unit? Whats a good way to diagnose?

[Nasty-LSX]

The reading has been off for a while. full only shows half a tank. When empty its at about a 1/4 tank
on gauge. What are your thoughts??

 

[Customblue]

Check your ground from the sending unit. IIRC, they are 0 ohms empty and 90 full. Fill her up and check ohms one lead on the ground the other on the supply to the gauge once you are certain the ground is good.