IF the gage is simply way off, you can calibrate it with a stick. Pull the sending unit out, then see what the fuel level is with the stick. (Do a little math to determine the percentage of full.) You can move the float up and down with it out (with the wires hooked up) to see if the gage is working, and adjust it to read the way you want. Mine is set up to leave 30 gallons in the tank on Empty.

Jeff

A powered up fuel gauge would always read full if the sender wire was shorted to the hull. If it isn't something simple and you check out the gauge using Doug's procedures. The next place to look is the sending unit That is the usual weak point in the fuel gauge system. The five bolt flush mounted ones are the easiest to replace with new Tempo units. These unit's are basically swingarm driven rheostats. In time the contact between the wiper and the resistive wire windings becomes worn and unreliable. If you have a four bolt sender mounted on top of a pipe it is a Rochester sender. It uses a magnet to drive a button gauge/sender that sits on top of the swingarm unit. New ones are available.

The other place to check is the back of the fuel gauge. If the wire leads are shorted there you will see a reading of full.

I am chasing the same gremlin. But my readings are too low.

I replaced my sender in the tank but the readings are still off.
I tried grounding the sender wire and it pegged the gauge as it should.

I am now going to try a jumper wire from the sender to the screw post on the back of the gauge to see if the readings change.
I bought the resistors that Doug suggested and will try that test as well.
Either the sender wire has a bad connection somewhere or the ground behind the gauge panel is faulty.
There is a terminal block in the wine cellar under the lower helm. The connections there might be corroded.

I will take pics and report back.

Consider using electronic senders: www.centroidproducts.com