Analog Fuel Gauge Problem

[FIUMario]

Recently purchased an 84 Mako 21' with an 88 Yamaha 200ETXG.
It has an 86 Gallon tank made by Fla Marine Tanks.

Fuel gauge would not pass 3/4 even with tank topped off.
Also, reading was sometimes static, not *****te (ex when the boat on the trailer the needle wouldn't move).
I replaced the Fuel Sender and it is better now. The needle went up to 7/8 when the tank is full and it reads more accurate now. If we shake the boat on the trailer the needle moves accordingly.

The reading is still off though, I can't get it to F when tank is full. What can be the problem? I am thinking one of these:

1. Gauge somehow (but how?) needs to be calibrated to F
2. Wiring is old, weak, not communicating the correct resistance - hence the improvement with the new sender with a stronger signal now.
3. Incorrect wiring? see attached pic: white on left, yellow on right, black on middle

I would really like to get this fixed, driving me crazy a bit!

Tks! Mario

[Dire Straits]

Is the boat level when you are taking a reading.

[hewesbayfisher90hp]

Other Ideas

Looks right - but is the center nut/lock secured all the way down or is it hitting the light retainer?

Does this gauge even read full when just shorted out, not sure what the range of values are - but think you follow what I say.

Then from there, you can see what the sender is reading, what range.

How is the sender positioned in the tank